Thin film acoustic resonator and method of producing the same

ABSTRACT

A pit ( 52 ) is formed in a substrate comprising a silicon wafer ( 51 ) on a surface of which a silicon oxide thin layer ( 53 ) is formed. A sandwich structure ( 60 ) comprising a piezoelectric layer ( 62 ) and lower and upper electrodes ( 61, 63 ) joined to both surfaces of the piezoelectric layer is disposed so as to stride over the pit ( 52 ). The upper surface of the lower electrode ( 61 ) and the lower surface of the piezoelectric layer ( 62 ) joined to the upper surface of the lower electrode are treated so that the RMS variation of the height thereof is equal to 25 nm or less. The thickness of the lower electrode ( 61 ) is set to 150 nm or less. According to such a structure, there is provided a high-performance thin film bulk acoustic resonator which are excellent in electromechanical coupling coefficient and acoustic quality factor.

This application claims priority benefits from Japanese PatentApplication No. 2001-141845 filed May 11, 2001 and Japanese PatentApplication No. 2001-141848 filed May 11, 2001 and Japanese PatentApplication No. 2001-182194 filed Jun. 15, 2001.

TECHNICAL FIELD

The present invention relates to a thin film bulk acoustic resonatorusing an electroacoustic effect of a piezoelectric thin film, and moreparticularly to a thin film bulk acoustic resonator usable as aconstituent element of a filter for electronic circuits of communicationequipment and a method of producing the same.

Further, the present invention relates to a device using a piezoelectricthin film used in a broad field such as a thin film oscillator, a thinfilm VCO (Voltage Control Oscillator), a thin film filter, a duplexer,various kinds of sensors, etc. used in mobile communication equipment orthe like.

BACKGROUND TECHNIQUE

In order to meet the need of reducing the cost and size of electronicequipment, an effort of reducing the size of a filter as a circuitconstituent element is being made continuously without changing its way.Strict requirements on both of the size and cost of constituent partsare imposed on consumer electronic equipment such as a cellular phone,miniature radio, etc. A circuit contained in such electronic equipmentuses filters which must be tuned precisely to predetermined frequencies.Accordingly, an effort of supplying inexpensive and compact filters isbeing continuously made at every moment.

Devices using the piezoelectric phenomenon have been used in variousfields. In the progress of miniaturization and power saving of portableequipment such as a cellular phone, etc., the application field ofsurface acoustic wave (SAW) devices as RF filters or IF filters used forthe above equipment is being enlarged. Enhancement of the design andproducing technologies of SAW filters have satisfied user's strictrequirements to specifications. However, as the frequencies being usedare shifted to a higher frequency band, the enhancement of thecharacteristics is approaching to its upper limit, so that greattechnical innovation has been required for both of the microstructure ofelectrodes to be formed and securement of stable output.

Further, a thin film acoustic resonator using the thickness vibration ofpiezoelectric thin film, that is, a thin film bulk acoustic resonator ora film bulk acoustic resonator (hereinafter referred to as “FBAR”) isconstructed by forming thin film mainly composed of piezoelectricelement and electrodes for driving the piezoelectric thin film on a thinsupport film formed on a substrate. FBAR thus constructed can performfundamental resonance in gigahertz band. If a filter is constructed byFBAR, the filter could be designed in a remarkably compact size, andalso it can be operated with low loss and in a broad band. In addition,it can be manufactured integrally with a semiconductor integratedcircuit. Therefore, it is expected that FBAR will be applied to futureultraminiature portable equipments.

A simple construction of the thin film bulk acoustic resonator has sucha sandwich structure that a piezoelectric (PZ) thin film material layeris sandwiched between two metal electrodes. The sandwich structure issupported by a bridge structure in which the peripheral portion thereofis supported and the center portion thereof is suspended in the air.When electric field is generated by applying a voltage across the twoelectrodes, the piezoelectric (PZ) thin film material converts some ofthe electrical energy to mechanical energy in the form of acoustic wave.The acoustic wave propagates in the same direction as the electricfield, and reflects at the interface between the electrode and the air.In the following description, the piezoelectric element may beabbreviated to PZ.

When mechanical resonance is induced, the thin film bulk acousticresonator serves as an electrical resonator due to the electricalenergy/mechanical energy converting property of the PZ thin filmmaterial. Accordingly, the filter can be constructed by the thin filmbulk acoustic resonator. The mechanical resonance of the thin film bulkacoustic resonator is induced at the frequency at which the thickness ofthe material through which the acoustic wave propagates is equal to ahalf of the wavelength of the acoustic wave. The frequency of theacoustic wave is equal to the frequency of an electrical signal appliedto the electrodes. The velocity of the acoustic wave is smaller than thevelocity of light by 5 to 6 figures, and thus the resonator achieved canbe made extremely compact. Therefore, the resonator to be used in theGHz band can be designed in the structure having a plane size less than200 micrometers and a thickness less than several micrometers.

In the thin film bulk acoustic resonator and stacked thin film acousticresonators having a plurality of the sandwich structures as describedabove, that is, stacked thin film bulk acoustic wave resonators andfilters (hereinafter referred to as “SBAR”), the center portion of thesandwich structure comprises piezoelectric thin film which ismanufactured to have a thickness of about 1 to 2 μm by the sputteringmethod. The upper and lower electrodes act as electrical leads and aredisposed so as to sandwich the piezoelectric thin film therebetween togive electrical field penetrating through the piezoelectric thin film.The piezoelectric thin film converts a part of the electric field energyto mechanical energy. In response to the applied electric field energywhich varies with time, the time-varying (stress/strain) energy isformed.

A piezoelectric thin film element applied to a resonator, a filter orthe like which uses such elastic wave is manufactured as follows.

By using various thin film forming methods, a base film comprising adielectric thin film, a conductive thin film or a stacked film of thedielectric thin film and the conductive thin film is formed on thesurface of a single crystal semiconductor substrate of silicon or thelike or on the surface of a substrate constructed by formingpolycrystalline diamond film or film of isoelastic metal such as elinvaror the like on silicon wafer or the like. Further, piezoelectric thinfilm is formed on the base film, and a desired upper structure isformed. After each film is formed or after all the films are formed,each film is subjected physical processing or chemical processing toperform micro-fabrication and patterning.

In order to operate FBAR or SBAR as a thin film bulk acoustic resonator,the sandwich structure containing the piezoelectric thin film must besupported by the bridge structure so that the air/crystal interface isformed to confine the acoustic wave in the sandwich structure. Thesandwich structure is generally constructed by stacking a lowerelectrode, a piezoelectric layer and an upper electrode in this order.Accordingly, the air/crystal interface has already existed at the upperside of the sandwich structure. In addition, the air/crystal interfacemust be also formed at the lower side of the sandwich structure. Inorder to achieve the air/crystal interface at the lower side of thesandwich structure, the following methods have been hitherto used.

According to a first method, as disclosed in JP(A)-58-153412 orJP(A)-60-142607 for example, etching away a part of wafer whichconstitutes a substrate is used. In the case where the substrate iscomposed of silicon, a part of the silicon substrate is etched away fromthe back side thereof by using heated KOH water solution to form a hole.That is, after a base film, a lower electrode, a piezoelectric thin filmand an upper electrode are formed on the upper surface of the substrate,a portion of the substrate located below a portion which will act as anoscillating portion is removed from the lower surface side of thesubstrate, thereby achieving a resonator having such a shape that theedge of the sandwich structure is supported by the silicon substrate ata portion surrounding the hole at the front surface side of the siliconsubstrate. However, the hole thus formed so as to penetrate through thewafer makes the wafer very fragile and easily breakable. Further, if wetetching using KOH is carried out at an etching inclination angle of 54.7degrees with respect to the surface of the substrate, it would bedifficult to increase the achievement amount of final products, that is,to increase the yield of FBAR/SBAR on wafer.

For example, the sandwich structure having a lateral dimension of about150 μm×150 μm (plane size) which is formed on silicon wafer having athickness of 250 μm needs a back-side etching hole of about 450 μm×450μm. Accordingly, about only one ninth of wafer can be used formanufacturing. After a portion of the substrate located below theoscillation portion of the piezoelectric thin film is removed byanisotropic etching to form a floating structure, the wafer is separatedevery element to achieve thin film bulk acoustic resonators (which arealso called as piezoelectric thin film resonators) corresponding to thePZ thin film elements.

According to a second method of providing the air/crystal interfacebelow the sandwich structure, as disclosed in JP(A)-2-13109, an airbridge type FBAR element is formed. Normally, a sacrificial layer isformed, and then a sandwich structure is formed on the sacrificiallayer. The sacrificial layer is removed at the end of the process orabout the end of the process. The overall processing is carried out atthe front surface side of wafer, and thus this method needs nopositioning at the both sides of the wafer and no large-area back-sideopening.

JP(A)-2000-69594 discloses the construction of an air bridge typeFBAR/SBAR and a method of manufacturing the same using phospho-silicateglass (PSG) as a sacrificial layer. In this publication, a PSG layer isdeposited on silicon wafer. PSG is deposited at a temperature of about450° C. or less by using silane and P₂O₂ to form soft-glass-likesubstance containing a phosphorus content of about 8%. PSG can bedeposited at a relatively low temperature, and it can be etched at avery high rate with dilute H₂O:HF water solution.

It is described in this publication that variation of RMS (root meansquare) of the height indicating the surface roughness of the PSGsacrificial layer is less than 0.5 μm, however, there is no descriptionon RMS variation of an order of 0.1 μm or less. The 0.1 μm-order RMSvariation is very roughly uneven at the atomic level. An FBAR/SBAR typethin film bulk acoustic resonator needs a piezoelectric material whosecrystal is grown as prismatic crystal vertical to the plane of theelectrode.

In JP(A)-2000-69594, it is described that a conductive sheet parallel tothe surface of the PSG layer is formed, and the RMS variation of theheight of the conductive sheet is less than 2 μm, however, there is nospecific description on the RMS variation of an order of 0.1 μm or less.The 0.1 μm-order RMS variation is an insufficient surface roughness forthe surface on which piezoelectric thin film for a thin film bulkacoustic resonator is formed. Various attempts to grow the piezoelectricthin film have been made. However, the crystal is grown in variousdirections due to the effect of various unevenness on the rough surface,so that the crystal quality of the piezoelectric thin film achieved isnot sufficient.

There is a method of providing a proper solid acoustic mirror in placeof provision of the air/crystal interface as described above. Accordingto this method, as disclosed in JP(A)-6-295181, a large acousticimpedance comprising an acoustic Bragg reflection mirror is formed belowthe sandwich structure. The Bragg reflection mirror is formed byalternately stacking layers of high and low acoustic impedancematerials. The thickness of each layer is fixed to one fourth of thewavelength of the resonant frequency. A sufficient number of layersenables the effective impedance at the interface between thepiezoelectric element and the electrode to be still higher than theacoustic impedance of the element. Accordingly, the acoustic wave in thepiezoelectric element can be confined effectively. An acoustic resonatorachieved according to this method is called as solid acoustic mirrormounted resonator (SMR) because there is no cavity below the sandwichstructure.

This method can avoid the problem of the first and second methods thatthere is formed such a film that the peripheral portion thereof is fixedand the center portion thereof is freely vibrated. However, this methodhas many problems. That is, since a metal layer forms a parasiticcapacitor which degrades the electrical performance of the filter, itcannot be used as the layer of the Bragg reflection mirror. Therefore,selection of materials usable for the Bragg reflection mirror isrestricted. The difference in acoustic impedance between layers formedof available materials is not large. Accordingly, a number of layers areneeded to confine the acoustic wave. Further, in this method, stressapplied to each layer must be controlled with high precision, and thusthe manufacturing process is complicated.

Further, it is difficult to form a viahole penetrating a large number oflayers such as the number of 10 to 14, and thus the acoustic resonatorsachieved according to this method are unfavorable to integration withother active elements. Further, according to examples which have beenever reported, the acoustic resonators achieved according to this methodhave effective coupling coefficients still lower than those of theacoustic resonators having air bridges. As a result, the filter based onSMR has a narrower effective band width than the filter using the airbridge type acoustic resonator.

As described above, in the thin film bulk acoustic resonator,(stress/strain) energy that varies with time in response to time-varyingapplied electric field energy is formed in the sandwich structure.Accordingly, when the adhesion force between the substrate and the lowerelectrode of the sandwich structure is low, the substrate and thesandwich structure are exfoliated from each other, so that durability islowered, that is, the lifetime of the thin film bulk acoustic resonatoris shortened.

In JP(A)-2000-69594, etc., Mo is described as proper electrode material.However, there is no specific description on enhancement of adhesionforce to silicon wafer, etc. serving as a substrate.

For example, JP(A)-2-309708 discloses that a lower electrode layercomprising two layers of Au/Ti or the like is used. In this case, the Tilayer exists as a layer for increasing the adhesion force between the Aulayer and the substrate. That is, the Ti adhesion layer is not anindispensable electrode layer from the viewpoint of the originaloperation of the thin film bulk acoustic resonator. However, when no Tiadhesion layer is formed and only the Au electrode layer is solelyformed, the adhesion force between the substrate and the Au electrodelayer is insufficient, and occurrence of exfoliation, etc. remarkablyreduces the durability of the thin film bulk acoustic resonator underoperation.

In the thin film bulk acoustic resonators as described above, thereexist not only required longitudinal vibration propagating in thedirection vertical to the electrode surface, but also lateral vibrationpropagating in the direction parallel to the electrode surface. Thelateral vibration contains a component that causes “spurious” in therequired vibration of the thin film bulk acoustic resonator to degradethe characteristics of the resonator.

An object of the present invention is to provide FBAR/SBAR havingimproved performance.

Another object of the present invention is to provide a high-performanceFBAR/SBAR that is excellent in electromechanical coupling coefficient,acoustic quality factor (Q-value), temperature characteristic, etc. byimproving the crystal quality of piezoelectric (PZ) thin film.

Another object of the present invention is to provide a high-performanceFBAR/SBAR that is excellent in electromechanical coupling coefficient,acoustic quality factor (Q-value), temperature characteristic, etc. bydevising the shape of an upper electrode.

Another object of the present invention is to provide a high-performanceFBAR/SBAR in which particularly spurious resonance is reduced.

Another object of the present invention is to improve durability ofFBAR/SBAR and thus improve the lifetime thereof by increasing theadhesion force (bonding strength) between a lower electrode layer and asubstrate.

Another object of the present invention is to provide a high-performanceFBAR/SBAR that is excellent in electromechanical coupling coefficient,acoustic quality factor (Q-value), etc. by increasing the adhesion forcebetween the lower electrode layer and the substrate and enablingformation of piezoelectric thin film having excellent crystal qualityand orientation on the lower electrode layer.

As the piezoelectric materials for piezoelectric thin film elements areused aluminum nitride (AlN), zinc oxide (ZnO), cadmium sulfide (CdS),lead titanate [PT] (PbTiO₃), lead titanate zirconate [PZT](Pb(Zr,Ti)O₃), etc. Particularly, AlN has higher propagation speed ofelastic wave, and it is suitably used as piezoelectric material ofpiezoelectric thin film resonator for a thin film bulk acousticresonator or filter which operate in a high-frequency band.

FBAR achieves resonance by elastic wave propagating in the thin film, sothat not only the vibration characteristic of the piezoelectric thinfilm, but also the vibration characteristics of the electrode layer andthe base film greatly affect the resonance characteristic of FBAR.Various attempts have been hitherto made to apply the AlN thin film toFBAR. However, there have not yet achieved any thin film bulk acousticresonator and any thin film filter which can exhibit sufficientperformance in GHz band. Accordingly, there has been strongly required apiezoelectric thin film resonator, that is, a thin film bulk acousticresonator in which the electromechanical coupling coefficient, acousticquality factor and the temperature stability of the resonant frequencyof a vibration portion containing not only the AlN thin film but also anelectrode layer and a base film are improved.

Therefore, the present invention has an object to provide apiezoelectric thin film resonator, that is, a thin film bulk acousticresonator in which the temperature stability of the resonant frequencyis improved without reducing the electromechanical coupling coefficientand acoustic quality factor by taking advantage of the characteristic ofthe AlN thin film with high propagation velocity of elastic wave.

SUMMARY OF THE INVENTION

In order to attain the above objects, according to the presentinvention, there is provided a thin film bulk acoustic resonator,comprising: a piezoelectric layer; a first electrode joined to a firstsurface of the piezoelectric layer; and a second electrode joined to asecond surface of the piezoelectric layer, which is located at theopposite side to the first surface, wherein RMS variation of the heightof the first surface of the piezoelectric layer is equal to 25 nm orless, preferably to 20 nm or less.

In the present invention, the RMS variation of the height means theroot-mean-square roughness: Rq described in Japanese Industrial StandardJIS B-0601: 2001 “Geometrical Properties Specification (GPS) ofProducts—Surface Property: Outlook Curved Line System—Term, Definitionand Surface Property Parameter” (the same is applied to the inventiondescribed below).

Further, according to the present invention, in order to achieve theabove object, there is provided a thin film bulk acoustic resonator,comprising: a piezoelectric layer; a first electrode joined to a firstsurface of the piezoelectric layer; and a second electrode joined to asecond surface of the piezoelectric layer, which is located at theopposite side to the first surface, wherein a surface of the firstelectrode facing the piezoelectric layer has RMS variation of the heightthereof that is equal to 25 nm or less, preferably to 20 nm or less.

In an aspect of the present invention, RMS variation of the height ofthe second surface of the piezoelectric layer is set to not more than 5%of the thickness of the piezoelectric layer. In an aspect of the presentinvention, waviness height of a surface of the second electrode is setto not more than 25% of the thickness of the piezoelectric layer.

In an aspect of the present invention, the second electrode has a centerportion and an outer peripheral portion having a larger thickness thanthe center portion. In an aspect of the present invention, the outerperipheral portion is located in a frame shape so as to surround thecenter portion. In an aspect of the present invention, the secondelectrode is designed so that thickness variation of the center portionis set to not more than 1% of the thickness of the center portion. In anaspect of the present invention, the thickness of the outer peripheralportion is set to not less than 1.1 time as high as the height of thecenter portion. In an aspect of the present invention, the outerperipheral portion is located within an area inwardly extending from anouter edge of the second electrode by a distance of 40 μm. In an aspectof the present invention, waviness height of a surface of the centerportion is set to not more than 25% of the thickness of thepiezoelectric layer.

In an aspect of the present invention, a sandwich structure comprisingthe piezoelectric layer, the first electrode and the second electrode issupported at an edge portion thereof by the substrate so as to strideover a pit or cavity or recess formed on a surface of the substrate. Inan aspect of the present invention, an insulating layer is formed on thesurface of the substrate so as to stride over the pit portion, and thesandwich structure is formed on the insulating layer.

Further, according to the present invention, in order to attain theabove object, there is provided a method of producing a thin film bulkacoustic resonator having a piezoelectric layer, a first electrodejoined to a first surface of the piezoelectric layer, and a secondelectrode joined to a second surface of the piezoelectric layer, whichis located at the opposite side to the first surface, comprising:forming a pit or cavity or recess on a surface of a substrate; fillingthe pit with a sacrificial layer; polishing a surface of the sacrificiallayer so that RMS variation of the height of the surface of thesacrificial layer is equal to 25 nm or less, preferably to 20 nm orless; forming the first electrode over a partial area of the surface ofthe sacrificial layer and a partial area of the surface of thesubstrate; forming the piezoelectric layer on the first electrode;forming the second electrode on the piezoelectric layer; and removingthe sacrificial layer from the inside of the pit by etching.

In an aspect of the present invention, the first electrode is formed ata thickness of 150 nm or less, and RMS variation of the height of anupper surface of the first electrode is set to 25 nm or less, preferablyto 20 nm or less.

In an aspect of the present invention, an insulating layer is formedbefore the first electrode is formed on the sacrificial layer.

Still further, according to the present invention, there is provided athin film bulk acoustic resonator, comprising: a substrate; and asandwich structure disposed on the substrate and having a piezoelectricthin film layer (piezoelectric layer), a lower electrode layer at thesubstrate side and an upper electrode layer paired with the lowerelectrode layer, which are stacked so that the piezoelectric thin filmlayer is sandwiched between the lower electrode layer and the upperelectrode layer, wherein the sandwich structure has an adhesionelectrode layer or adherence electrode layer located between the lowerelectrode layer and the substrate and joined to the lower electrodelayer, and the adhesion electrode layer is joined to the substratearound a pit or cavity or recess which is formed on the substrate so asto permit vibration of the sandwich structure.

In an aspect of the present invention, the adhesion electrode layer isformed in an annular shape, and when a plane area of a portion of theadhesion electrode layer which is brought into contact with the lowerelectrode layer is represented by S1 and a plane area of the lowerelectrode layer is represented by S2, the relationship:0.01×S2≦S1≦0.5×S2 is satisfied, and the upper electrode layer is locatedin an area corresponding to the inside of the adhesion electrode layer.

In an aspect of the present invention, the adhesion electrode layercomprises material containing at least one material selected from agroup consisting of Ti, Cr, Ni and Ta, the lower electrode layercomprises material containing at least one material selected from agroup consisting of Au, Pt, W and Mo, and the piezoelectric thin filmlayer is formed of AlN or ZnO.

Still further, according to the present invention, there is provided amethod of producing the thin film bulk acoustic resonator, comprising:forming the adhesion electrode layer on the surface of the substratehaving the pit so that the adhesion electrode layer is located aroundthe pit; forming a sacrificial layer on the surface of the substrate sothat the sacrificial layer is located in an area corresponding to thepit located at the inside of the adhesion electrode layer; polishing asurface of the sacrificial layer so that RMS variation of the heightthereof is equal to 25 nm or less, preferably to 20 nm or less;successively forming the lower electrode layer, the piezoelectric thinfilm layer and the upper electrode layer on the sacrificial layer andthe adhesion electrode layer; and then removing the sacrificial layer.

In an aspect of the present invention, the sacrificial layer is formedby forming a layer of sacrificial layer material so that the substrateand the adhesion electrode layer are covered by the layer of thesacrificial layer material, and polishing the layer of the sacrificiallayer material so that the surface of the adhesion electrode layer isexposed, and the sacrificial layer is removed by etching, and glass orplastic material is used for the sacrificial layer.

The inventors have discovered that the temperature stability ofresonance frequencies can be improved with keeping highelectromechanical coupling coefficient and high acoustic quality factorby forming electrodes on both the surfaces of a piezoelectric thin filmwhich is mainly formed of AlN, the electrodes being mainly formed ofmolybdenum (Mo) that has larger elastic modulus than general electrodematerials such as gold, platinum, aluminum, copper, etc. and remarkablysmall thermal elasticity loss, and further forming an insulating layermainly containing silicon oxide (SiO₂) or silicon nitride (Si₃N₄) havinga temperature coefficient of resonant frequency whose sign is differentfrom the temperature coefficient of resonant frequency of thepiezoelectric thin film while the insulating layer is contained in thevibration portion, and have implemented the present invention on thebasis of this discovery. Further, the inventors have also discoveredthat high-performance FBAR having high electromechanical couplingcoefficient and high acoustic quality factor and remarkably excellenttemperature stability can be implemented by setting the thickness ofeach layer so as to satisfy 0.1≦t′/t≦0.5, preferably 0.2≦t′/t≦0.4wherein the thickness of the piezoelectric thin film mainly formed ofaluminum nitride is represented by t and the thickness of the insulatinglayer mainly formed of silicon oxide or silicon nitride (when there areplural insulating layers, “thickness” means the total thickness of therespective insulating layers) is represented by t′.

That is, according to the present invention, in order to attain theabove object, there is provided a piezoelectric thin film resonator,comprising: a substrate; and a piezoelectric stack structure formed onthe substrate in which a vibrating portion is constructed to contain apart of the piezoelectric stack structure, the piezoelectric stackstructure is formed by stacking a lower electrode, a piezoelectric film(piezoelectric layer) and an upper electrode in this order from thesubstrate side, and a cavity for permitting vibration of the vibratingportion is formed in the substrate in an area corresponding to thevibrating portion, wherein the piezoelectric film is mainly formed ofaluminum nitride, the lower electrode and the upper electrode are mainlyformed of molybdenum, and the vibrating portion contains at least a partof at least one insulating layer mainly formed of silicon oxide orsilicon nitride which is joined to the piezoelectric stack structure. Itshould be noted that the terms “piezoelectric thin film resonator” and“thin film bulk acoustic resonator” have the same meaning in thisspecification.

In an aspect of the present invention, the thickness t of thepiezoelectric film and the total thickness t′ of at least one insulatinglayer satisfies the following inequality: 0.1≦t′/t≦0.5. Here, in case ofone insulating layer, the total thickness t′ means the thicknessthereof, and, in case of plural insulating layers, the total thicknesst′ means the sum of the thickness of the respective insulating layers.

In an aspect of the present invention, the content of aluminum nitridein the piezoelectric film is set to 90 equivalent % or more. In anaspect of the present invention, the content of silicon oxide or siliconnitride in the insulating layer is set to 50 equivalent % or more. In anaspect of the present invention, the content of molybdenum in each ofthe lower electrode and the upper electrode is set to 80 equivalent %(atomic %) or more.

In an aspect of the present invention, one of the at least oneinsulating layer is formed on the surface of the substrate. In an aspectof the present invention, one of the at least one insulating layer isformed on a surface of the piezoelectric stack structure at the oppositeside to the substrate.

In an aspect of the present invention, the substrate is formed of singlecrystal silicon. In an aspect of the present invention, the upperelectrode comprises a first electrode portion and a second electrodeportion that are formed so as to be spaced from each other.

In an aspect of the present invention, the electromechanical couplingcoefficient determined on the basis of the measurement values of theresonance frequency and the antiresonance frequency in the neighborhoodof 2.0 GHz is equal to 4.0 to 6.5%, and the acoustic quality factorthereof is equal to 750 to 2000, and the temperature coefficient of theresonance frequency is equal to −20 to 20 ppm/° C.

Further, according to the present invention, there are provided a VCO(voltage control oscillator), a filter and a transmission/receptionduplexer which are constructed by using the piezoelectric thin filmresonator described above, and the characteristics can be remarkablyimproved at high frequencies of 1 GHz or more in these devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the basicconstruction of FBAR which is a thin film bulk acoustic resonatoraccording to the present invention;

FIG. 2 is a schematic cross-sectional view showing the basicconstruction of SBAR which is a thin film bulk acoustic resonatoraccording to the present invention;

FIG. 3 is a schematic cross-sectional view showing a method of producingFBAR (thin film bulk acoustic resonator) according to the presentinvention, and an embodiment of FBAR thus achieved;

FIG. 4 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment FBAR thus achieved;

FIG. 5 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment of FBAR thus achieved;

FIG. 6 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment of FBAR thus achieved;

FIG. 7 is a schematic plan view showing the method of producing FBAR(thin film bulk acoustic resonator) according to the present invention,and the embodiment of FBAR thus achieved;

FIG. 8 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment of FBAR thus achieved;

FIG. 9 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment of FBAR thus achieved;

FIG. 10 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment of FBAR thus achieved;

FIG. 11 is a schematic cross-sectional view showing the method ofproducing FBAR (thin film bulk acoustic resonator) according to thepresent invention, and the embodiment of FBAR thus achieved;

FIG. 12 is a plan view showing the method of producing FBAR (thin filmbulk acoustic resonator) according to the present invention, and anupper electrode of FBAR thus achieved;

FIG. 13 is a schematic cross-sectional view showing FBAR according tothe present invention;

FIG. 14 is a schematic cross-sectional view showing SBAR according tothe present invention;

FIG. 15 is a schematic cross-sectional view showing FBAR and a method ofproducing FBAR according to the present invention;

FIG. 16 is a schematic cross-sectional view showing FBAR and the methodof producing FBAR according to the present invention;

FIG. 17 is a schematic cross-sectional view showing FBAR and the methodof producing FBAR according to the present invention;

FIG. 18 is a schematic cross-sectional view showing FBAR and the methodof producing FBAR according to the present invention;

FIG. 19 is a schematic cross-sectional view showing FBAR and the methodof producing FBAR according to the present invention;

FIG. 20 is a schematic cross-sectional view showing FBAR and the methodof producing FBAR according to the present invention;

FIG. 21 is a schematic plan view showing FBAR and the method ofproducing FBAR according to the present invention;

FIG. 22 is a schematic plan view showing an embodiment of a thin filmbulk acoustic resonator according to the present invention;

FIG. 23 is a cross-sectional view taken along X—X of FIG. 22;

FIG. 24 is a schematic plan view showing an embodiment of a thin filmbulk acoustic resonator according to the present invention;

FIG. 25 is a cross-sectional view taken along X—X of FIG. 24;

FIG. 26 is a schematic plan view showing an embodiment of a thin filmbulk acoustic resonator according to the present invention;

FIG. 27 is a cross-sectional view taken along X—X of FIG. 26;

FIG. 28 is a schematic plan view showing an embodiment of a thin filmbulk acoustic resonator according to the present invention; and

FIG. 29 is a cross-sectional view taken along X—X of FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedescribed hereunder with reference to the accompanying drawings.

FIGS. 1 and 2 are schematic cross-sectional views showing the basicconstruction of each of FBAR and SBAR which is a thin film bulk acousticresonator according to the present invention.

In FIG. 1, FBAR 20 comprises an upper electrode 21 and a lower electrode23, and a piezoelectric (PZ) material layer 22 which is partiallysandwiched between the upper and lower electrodes 21 and 23, therebyforming a sandwich structure. Aluminum nitride (AlN) or zinc oxide (ZnO)is preferably used as the PZ material. The electrodes 21, 23 used forFBAR 20 are preferably formed of molybdenum, however, they may be formedof other materials.

This device (FBAR) utilizes the action of bulk elastic acoustic wave inthe thin film of PZ material. When electric field is generated betweenthe two electrodes 21, 23 by applying a voltage across them, the PZmaterial converts a part of electrical energy to mechanical energy inthe form of acoustic wave. The acoustic wave propagates in the samedirection as the electric field, and is reflected at the electrode/airinterface.

When mechanically resonating, the acoustic resonator plays a role as anelectrical resonator by the electrical energy/mechanical energyconverting property of the PZ material. Accordingly, the device canoperate as a notch filter. The mechanical resonance of the device occursat the frequency at which the thickness of the material through whichthe acoustic wave propagates is equal to the half wavelength of theacoustic wave. The frequency of the acoustic wave is equal to thefrequency of an electrical signal applied between the electrodes 21, 23.Since the velocity of sound is smaller than the velocity of light byfive to six figures, the resonator thus achieved can be designed to beextremely compact. The resonators to be applied in the GHz band can beconstructed in the plane size of about 100 μm order and in the thicknessof several micrometers order.

Next, SBAR will be described with reference to FIG. 2. SBAR 40 has anelectrical function similar to a band filter. SBAR 40 basicallycorresponds to two mechanically-coupled FBARs. A signal traversing theelectrodes 43 and 44 at the resonance frequency of the piezoelectriclayer 41 transmits the acoustic energy to the piezoelectric layer 42.The mechanical vibration in the piezoelectric layer 42 is converted toan electrical signal traversing the electrodes 44 and 45.

FIGS. 3 to 8 are schematic cross-sectional views (FIGS. 3 to 6 and 8)and a schematic plan view (FIG. 7) showing a method of producing ormanufacturing FBAR (thin film bulk acoustic resonator) and an embodimentof FBAR thus achieved.

First, as shown in FIG. 3, a pit or cavity or recess is formed byetching in a typical silicon wafer 51 used for manufacturing anintegrated circuit. The depth of the pit is preferably set to 1.5 to 30μm, more preferably to 1.5 to 10 μm or to 3 to 30 μm as occasiondemands. It is sufficient that the depth of a cavity below the sandwichstructure of FBAR may be set to several micrometers in consideration ofthe fact that the cavity is merely used to allow displacement induced inthe piezoelectric layer.

A thin layer 53 of silicon oxide is formed on the surface of the wafer51 by thermal oxidation so that phosphorus of a sacrificial layer of PSGwhich will be formed on the thin layer 53 in a subsequent step isprevented from being diffused into the wafer 51. In place of the siliconoxide layer, a silicon nitride layer formed by the low pressure CVDmethod may be used as the thin layer 53. By suppressing the diffusion ofphosphorus into the wafer as described above, the silicon wafer can beprevented from being converted to conductive material, and thus theelectrical action of the device thus achieved can be prevented frombeing adversely effected. The wafer 51 on which the thin layer 53 ofsilicon oxide or silicon nitride is formed is used as the substrate.That is, FIG. 3 shows a state that a pit or cavity or recess 52 isformed on the surface of the substrate to have a depth of preferably 1.5to 30 μm, more preferably 1.5 to 10 μm or 3 to 30 μm as occasiondemands.

Next, as shown in FIG. 4, a phospho-silicate glass (PSG) layer 55 isdeposited on the thin layer 53 of silicon oxide or silicon nitride ofthe substrate. PSG is deposited at a temperature of about 450° C. orless by using silane and P₂O₅ source as raw materials to formsoft-glass-like material containing about 8% phosphorus. As examples ofsilane may be used monosilane (SiH₄), trichlorosilane (SiHCl₃), silicontetramethoxide (Si(OCH₃)₄), silicon tetraethoxide (Si(OC₂H₅)₄), etc. Asexamples of the material serving as the P₂O₅ source may be usedphosphine (PH₃), trimethyl phosphite (P(OCH₃)₃), triethyl phosphite(P(OC₂H₅)₃), trimethyl phosphate (PO(OCH₃)₃), triethyl phosphate(PO(OC₂H₅)₃), etc. as well as P₂O₅.

This low-temperature process is well known to persons skilled in theart. Since PSG is a very clean inactive material which can be depositedat a relatively low temperature and etched at a very high etching ratewith dilute H₂O:HF solution, it is preferably used as the material ofthe sacrificial layer. In the etching step executed in the subsequentprocess, an etching rate of about 3 μm per minute can be achieved at adilute ratio of 10.

The surface of the PSG sacrificial layer 55 thus deposited would be veryrough at the atomic level if it is left as it is. Accordingly, the PSGsacrificial layer 55 as deposited is insufficient as a base body to forman acoustic resonator. An FBAR/SBAR type bulk acoustic resonator needspiezoelectric material whose crystal is grown with forming prismatic orcolumnar crystal vertical to the electrode surface. The surface of thePSG sacrificial layer 55 is polished to be smoothened with use ofpolishing slurry containing fine polishing particles to form a thin filmof piezoelectric material having excellent crystallinity andorientation.

That is, by polishing the surface of the PSG layer 55 with roughlyfinishing slurry as shown in FIG. 5, the overall surface of the PSGlayer 55 is flattened or planarized and a portion of the PSG layer atthe outside of the pit 52 is removed. Subsequently, the remaining PSGlayer 55 is further polished with slurry containing fine polishingparticles. As a substitute method, the above two polishing steps may beexecuted by using one fine slurry if further polishing time is allowed.The target is mirror-like finishing.

In the present invention, before the PSG layer is polished, it ispreferable to subject the PSG layer to heat treatment at hightemperature for the purpose of densification and reflow. The heattreatment of the PSG layer may be performed by RTA (Rapid ThermalAnneal) method. This is carried out at a temperature range from 750° C.to 950° C. under nitrogen atmosphere or nitrogen-oxygen mixtureatmosphere. Alternatively, the high-temperature heat treatment may becarried out in a diffusion furnace or by lamp heating. In the presentinvention, by subjecting the PSG layer to the high-temperature heattreatment, the PSG layer can be designed to have a more densifiedstructure and the hardness thereof is increased. By increasing thehardness of the PSG layer, polishing damage such as scratch or the likeis prevented from occurring on the surface of the PSG film in thesubsequent CMP (Chemical Mechanical Polishing), and the surface of thePSG film can be excellently flattened.

It is important to clean the substrate on which the PSG layer 55 remainsat the position corresponding to the pit 52 as described above. Theslurry causes a slight amount of silica coarse grain to remain on thewafer. Accordingly, the coarse grain must be removed. In a preferredembodiment of the present invention, the removal of the coarse grain iscarried out by using a second polishing tool having a rigid pad such asPolytex (trademark) produced by Rodel Nitta Incorporated Company. Inthis case, deionized water is used as lubricant, and the wafer isimmersed in the deionized water from the time when the substrate hasbeen polished until the time when the standby of the final cleaning stepis completed. It should be noted that the substrate is prevented frombeing dried from the last polishing step to the final cleaning step. Thelast cleaning step comprises a step of immersing the substrate into aseries of tanks which are charged with various chemicals. Ultrasonicstirring is applied to each tank. Such cleaning means is known topersons skilled in the art.

The polishing material is formed of silica fine particles. In thepreferred embodiment of the present invention, ammonia-based slurry ofsilica fine particle (Rodel Klebosol#30N: Rodel Nitta IncorporatedCompany) is used as the polishing material.

In the foregoing description, the specific polishing and cleaningmanners are described. However, any polishing and cleaning manners maybe used insofar as they can give required smooth surface. In thepreferred embodiment of the present invention, with respect to thefinished surface, the RMS variation of the height of the surface whichis measured by an atomic force microscope probe is equal to 25 nm orless (preferably 20 nm or less).

After the surface is cleaned as described above, a lower electrode 61 ofthe sandwich structure 60 is deposited as shown in FIG. 6. Molybdenum(Mo) is a preferable material for the lower electrode 61. However, thelower electrode 61 may be formed of other material such as Al, W, Au, Pror Ti. Molybdenum (Mo) is preferable because it has low thermoelasticloss. For example, the thermoelastic loss of Mo is equal to about{fraction (1/56)} of that of Al.

The thickness of the lower electrode 61 is important. A thick layer hasa rougher surface than a thin layer. It is important for the performanceof the resonator achieved to keep the smooth surface for deposition ofthe piezoelectric layer 62. Accordingly, the thickness of the lowerelectrode is preferably equal to 150 nm or less. Mo is preferablydeposited by sputtering. In the above step is achieved an Mo layer inwhich the RMS variation of the height of the surface is equal to 25 nmor less (preferably 20 nm or less).

After the deposition of the lower electrode 61 is finished, thepiezoelectric layer 62 is deposited. The preferable material of thepiezoelectric layer 62 is AlN or ZnO, and it is deposited by sputtering.In the preferred embodiment of the present invention, the thickness ofthe piezoelectric layer 62 is set to a value in the range from 0.1 μm to10 μm (preferably 0.5 μm to 2 μm). With respect to the upper surface ofthe piezoelectric layer 62, the RMS variation of the height ispreferably equal to 5% or less of the thickness (average value) of thepiezoelectric layer.

Finally, an upper electrode 63 is deposited. The upper electrode 63 isformed of the same material as the lower electrode 61, and it ispreferably formed of Mo.

After the sandwich structure 60 that comprises the lower electrode 61,the piezoelectric layer 62 and the upper electrode 63 and is patternedin a desired shape as described above is formed, the etching usingdilute H₂O:HF solution is started from the edge portion of the sandwichstructure 60 or a portion of the sacrificial layer 55 which is notcovered by the sandwich structure 60 and thus exposed to the outside asshown in FIG. 7, thereby removing PSG below the sandwich structure 60.Accordingly, the sandwich structure 60 suspended over the pit 52 remainsas shown in FIG. 8. That is, the edge portion of the sandwich structure60 is supported on the substrate so as to stride over the pit 52 formedon the surface of the substrate.

In FBAR thus achieved, according to the surface of the sacrificial layer55 (the RMS variation of the height is equal to 25 nm or less(preferably 20 nm or less)), the RMS variation of the height of thelower surface of the lower electrode 61 formed on the sacrificial layer55 is equal to 25 nm or less (preferably 20 nm or less). Further, sincethe thickness of the lower electrode 61 is small, the RMS variation ofthe height of the upper surface thereof is equal to 25 nm or less(preferably 20 nm or less). According to the upper surface of the lowerelectrode 61, the RMS variation of the height of the lower surface of apiezoelectric layer 62 formed on the upper surface of the lowerelectrode 61 is equal to 25 nm or less (preferably 20 nm or less).Although the smooth upper surface of the lower electrode 61 has nocrystal structure serving as nucleation site for growth of thepiezoelectric layer 62 formed thereon, it forms very regular c-axisorientation in the piezoelectric layer 62 thus formed, and givesexcellent piezoelectric characteristics to the piezoelectric layer 62.

FIGS. 9 to 10 are schematic cross-sectional views showing another methodof manufacturing FBAR which is a thin film bulk acoustic resonatoraccording to the present invention and another embodiment of FBAR thusachieved.

In this embodiment, after the steps described with reference to FIGS. 3to 5, an insulating layer 54 is formed as shown in FIG. 9. Theinsulating layer 54 is formed of, for example, SiO₂ film, and it may bedeposited by the CVD method. In consideration of resistance to etchingfluid to remove the sacrificial layer 55, Si₃N₄ film formed by thelow-pressure CVD method is more preferably used as the insulating layer54 than SiO₂ film. When SiO₂ film is used as the insulating layer 54, aproper protect is applied to the exposed surface of the SiO₂ film whenthe etching is carried out to remove the sacrificial layer 55.

Subsequently, the sandwich structure 60 is formed thereon by carryingout the step described with respect to FIG. 6. Subsequently, the stepsdescribed with reference to FIGS. 7 and 8 are carried out to achieveFBAR as shown in FIG. 10. At this time, in order to etch away thesacrificial layer 55, an opening having a proper size is formed at theedge portion of the sandwich structure 60 or a portion of the insulatinglayer 54 which is not covered by the sandwich structure 60 and locatedabove the sacrificial layer 55, etching fluid is supplied to thesacrificial layer 55 through the opening.

In FBAR of this embodiment, the insulating layer 54 is disposed betweenthe sandwich structure 60 and the cavity 52, and the vibrating portionis constructed to contain the insulating layer 54 as well as thesandwich structure 60, so that the strength of the vibrating portion isincreased and further the frequency temperature characteristic in thevibration of the vibrating portion can be improved.

The thickness t′ of the insulating layer 54 is preferably set to a valuein the range from 50 to 1000 nm. This is because the ratio (t′/t) of thethickness t′ of the insulating layer 54 to the thickness t of thepiezoelectric layer 62 is preferably set to a value in the range from0.1 to 0.5, and the thickness t of the piezoelectric layer 62 ispreferably set to a value in the range from 500 nm to 2000 nm. Thereason why the ratio t′/t is preferably set to a value in the range from0.1 to 0.5 is as follows. That is, if t′/t is set to 0.1 or more, theeffect of improving the frequency temperature characteristic in thevibration of the vibrating portion containing the insulating layer 54would be enhanced. If t′/t is set to 0.5 or less, reduction of theelectromechanical coupling coefficient and the acoustic quality factor(Q value) can be prevented. The RMS variation of the height of the uppersurface of the insulating layer 54 is equal to 25 nm or less (preferably20 nm or less).

In order to achieve a higher acoustic quality factor (Q value) in theabove embodiment, the uniformity of the thickness of each of theinsulating layer 54, the lower electrode 61, the piezoelectric layer 62and the upper electrode 63 is required to be further excellent. Thewaviness height of the surface of the upper electrode 63 reflects theabove uniformity of the thickness (that is, large waving height of thesurface of the upper electrode 63 shows low thickness uniformity of atleast one of the above layers, etc.). Therefore, in order to achieve afurther higher acoustic quality factor (Q value), the waviness height ofthe surface of the upper electrode 63 is preferably set to 25% or lessof the piezoelectric layer 62. From another viewpoint, the wavinessheight of the surface of the upper electrode 63 is preferably set to notmore than 0.5% of the measurement length (when the measurement length isequal to 150 μm, the waviness height is equal to 0.75 μm or less).

The above embodiments relates to FBAR. However, it is clear that fromthe foregoing description persons skilled in the art can produce SBAR byusing the same process. In the case of SBAR, another piezoelectric layer(second piezoelectric layer) and an upper electrode (second upperelectrode) on the second piezoelectric layer must be deposited. Sincethe second piezoelectric layer is formed on the upper electrode of“FBAR” shown in the above embodiment, the upper electrode must be alsokept to be 150 nm or less in thickness to form a surface which is soexcellent that the second piezoelectric layer is deposited on thesurface (i.e. the same level as the surface of the lower electrode forthe first piezoelectric layer).

FIG. 11 is a schematic cross-sectional view showing another method ofmanufacturing FBAR which is a thin film bulk acoustic resonatoraccording to the present invention and another embodiment of FBAR thusachieved, and FIG. 12 is a plan view of the upper electrode of FBAR ofFIG. 11. This embodiment is different from the embodiment shown in FIGS.3 to 8 only in the shape of the upper electrode 63.

In this embodiment, the upper electrode 63 has a center portion 631 andan outer peripheral portion 632 having a thickness larger than that ofthe center portion 631. The outer peripheral portion 632 is located inthe form of a frame around the center portion 631. The boundary betweenthe center portion 631 and the outer peripheral portion 632 is formed bya step.

The thickness of the outer peripheral portion 632 is preferably 1.1 timeor more as large as the thickness of the center portion 631. Thethickness variation of the center portion 631 is preferably set to notmore than 1% of the thickness (average value) of the center portion. Thesize “a” of the upper electrode 63 is set to 100 μm, for example. Theouter peripheral portion 632 is located within an area extendinginwardly from the outer edge of the upper electrode 63 at a distance“b”. The distance “b” is set to 40 μm or less, for example.

By providing the upper electrode with the above structure, occurrence ofvibration in the lateral direction at the outer peripheral portion ofthe upper electrode can be prevented, and extra spurious vibration isprevented from being superposed on the vibration of the acousticresonator. As a result, the resonance characteristic and the qualityfactor of the acoustic resonator and filter can be improved.

In this embodiment, in order to achieve a further higher acousticquality factor (Q value), the waviness height of the surface of thecenter portion 631 of the upper electrode 63 is preferably set to notmore than 25% of the thickness of the piezoelectric layer 62. Further,from another viewpoint, the waviness height of the surface of the centerportion 631 of the upper electrode 63 is preferably set to not more than0.5% of the measurement length.

In the above-described embodiments of the present invention, thesacrificial layer constructed by PSG is used. However, another materialmay be used for the sacrificial layer. For example, BPSG(Boron-Phosphor-Silicate-Glass) or another type glass such as spin glassor the like may be used. Further, plastics such as polyvinyl,polypropylene and polystyrene which can be deposited on suitablematerial by spinning may be used. The surfaces of these materialsdeposited are not smooth at the atomic level. Therefore, when thesacrificial layer is formed of each of these materials, the surfacesmoothening based on polishing is important as in the case of the PSGsacrificial layer. These sacrificial layers may be removed by organicremoving materials or O₂ plasma etching.

FIGS. 13 and 14 are cross-sectional views showing FBAR and SBAR whichare a thin film bulk acoustic resonator according to the presentinvention, respectively.

In FIG. 13, FBAR 20 comprises an upper electrode layer 21, a lowerelectrode layer 23 and an adhesion electrode layer or adherenceelectrode layer 24, and a part of a piezoelectric thin film layer 22 issandwiched between these layers to form a sandwich structure. Aluminumnitride (AlN) or zinc oxide (ZnO) is used as preferable material for thepiezoelectric thin film layer 22. The adhesion electrode layer 24 usedfor FBAR 20 is preferably formed of Ti, Cr, Ni or Ta, however, it may beformed of other materials. The upper and lower electrode layers 21, 23are preferably formed of Au, Pt, W or Mo, however, other materials maybe used. The sandwich structure is disposed so that the adhesionelectrode layer 24 is located on the substrate 11 around the pit 12formed on the upper surface of the substrate 11.

This device utilizes the action of bulk elastic acoustic wave in thepiezoelectric thin film layer. When electrical field is generatedbetween two electrodes 21, 23 by applying a voltage across theelectrodes, the piezoelectric thin film converts a part of electricalenergy to mechanical energy in the form of acoustic wave. The acousticwave propagates in the same direction as the electric field, and isreflected at the electrode/air interface.

When mechanically resonating, the acoustic resonator play a role as anelectrical resonator by the electrical energy/mechanical energyconverting property of the PZ material. Accordingly, the device canoperate as a notch filter. The mechanical resonance of the device occursat the frequency at which the thickness of the material through whichthe acoustic wave propagates is equal to the half wavelength of theacoustic wave. The frequency of the acoustic wave is equal to thefrequency of an electrical signal applied between the electrodes 21, 23.Since the velocity of sound is smaller than the velocity of light byfive to six figures, the resonator thus achieved can be designed to beextremely compact. The resonators to be applied in the GHz band can beconstructed in the plane size of about 100 μm order and in the thicknessof several micrometers order.

Next, SBAR will be described with reference to FIG. 14. SBAR 40 gives anelectrical function similar to that of a band filter. SBAR 40 basicallycorresponds to two mechanically-coupled FBARs. A signal traversing theadhesion electrode layer 24, the lower electrode layer 45 and theelectrode layer 44 at the resonance frequency of the piezoelectric thinfilm layer 42 transmits the acoustic energy to the piezoelectric thinfilm layer 41. The mechanical vibration in the piezoelectric thin filmlayer 41 is converted to an electrical signal traversing the electrodelayer 44 and the electrode 43.

FIGS. 15 to 21 are schematic cross-sectional views (FIGS. 15 to 20) anda schematic plan view (FIG. 21) showing a method of producing ormanufacturing FBAR (thin film bulk acoustic resonator) according to thepresent invention and an embodiment of FBAR thus achieved.

First, as shown in FIG. 15, a pit or cavity or recess is formed on anormal silicon wafer 51 used for manufacturing integrated circuits byetching. The depth of the pit is preferably set to 1.5 to 30 μm, morepreferably to 1.5 to 10 μm or to 3 to 30 μm as occasion demands. Thedepth of a cavity below the sandwich structure of FBAR is set so thatthe displacement generated by the piezoelectric thin film layer ispermitted. Accordingly, the depth of the cavity may be set to severalmicrometers at maximum.

A thin layer 53 of silicon oxide is formed on the surface of the wafer51 by thermal oxidation so that phosphorus is prevented from beingdiffused into the wafer 51 from the sacrificial layer of PSG which willbe formed on the silicon oxide thin layer 53 in the subsequent step. Inplace of the silicon oxide layer, a silicon nitride layer formed by thelow-pressure CVD method may be used as the thin layer 53. By suppressingthe diffusion of phosphorus into the wafer 51 as described above, thesilicon wafer is prevented from being converted to a conductor and theadverse effect on the electrical action of the device thus manufacturedcan be eliminated. The wafer 51 on which the thin layer 53 of siliconoxide or silicon nitride is formed as described above is used as thesubstrate. That is, FIG. 15 shows a state that a pit or cavity or recess52 having a depth which is preferably set to 1.5 to 30 μm, morepreferably to 1.5 to 10μm or to 3 to 30 μm as occasion demands is formedon the surface of the substrate.

Next, as shown in FIG. 16, an adhesion electrode layer or adherenceelectrode layer 161 is joined to and formed on the substrate so as tosurround the pit 52. Representing the area (plane area) of the uppersurface of the adhesion electrode layer 161 by S1 and also representingthe plane area of the lower electrode formed on the adhesion electrodelayer 161 by S2, it is preferable that S1 and S2 satisfies the followinginequality: 0.01×S2≦S1≦0.5×S2. When S1<0.01×S2, there is a tendency thatthe adhesion force between the substrate and the lower electrode isweaker and a sufficient effect of the present invention cannot beachieved. Further, when S1>0.5×S2, the adhesion electrode layer 161affects the action of the thin film bulk acoustic resonator, and thusthere is a tendency that an excellent resonance characteristic cannot beachieved. The thickness of the adhesion electrode layer 161 may be setto such a level that it can sufficiently hold the lower electrode layerto be formed on the adhesion electrode layer 161. For example, it may beset to any value in the range from 20 nm to 1 μm. The constituentmaterial of the adhesion electrode layer 161 may contain at least onematerial selected from the group consisting of Ti, Cr, Ni and Ta.

By providing the adhesion electrode layer 161 around the pit 52 of thesubstrate 52 as described above, occurrence of vibration in the lateraldirection in the thin film bulk acoustic resonator can be suppressed,and extra spurious vibration is prevented from being superposed on thevibration of the thin film bulk acoustic resonator. As a result, theresonance characteristics and the quality factors of the thin film bulkacoustic resonator and the filter can be improved.

Further, the adhesion electrode layer 161 formed of Ti, Cr, Ni, Ta orthe like does not exist at the lower side of the center portion of thelower electrode layer formed of Au, Pt, W, Mo or the like, and thus theorientation and crystallinity of the lower electrode layer can beimproved at this portion. As a result, there can be achieved apiezoelectric thin film layer that has a small FWHM (Full Width at HalfMaximum) of diffraction peak at a rocking curve and excellentorientation and crystalline quality. The high orientation and theexcellent crystallinity of the piezoelectric thin film layer improvesthe resonance characteristics and the quality factors of the thin filmbulk acoustic resonator and the filter according to the presentinvention.

Subsequently, a sacrificial layer 55 of PSG is deposited on the thinlayer 53 of silicon oxide or silicon nitride of the substrate on whichthe adhesion electrode layer 161 is formed. As described above, PSG isdeposited at a temperature of about 450° C. or less by using silane andP₂O₅ source as raw materials as described above to thereby form asoft-glass-like material containing about 8% phosphorus. As examples ofsilane may be used monosilane (SiH₄), trichlorosilane (SiHCl₃), silicontetramethoxide (Si(OCH₃)₄), silicon tetraethoxide (Si(OC₂H₅)₄), etc. Asexamples of the P₂O₅ source may be used not only P₂O₅, but alsophosphine (PH₃), trimethyl phosphite (P(OCH₃)₃), triethyl phosphite(P(OC₂H₅)₃), trimethyl phosphate (PO(OCH₃)₃), triethyl phosphate(PO(OC₂H₅)₃), etc. This low-temperature process is well known to personsskilled in the art.

PSG is a very clean inactive material which can be deposited at arelatively low temperature and etched at a very high etching rate withdilute H₂O:HF solution, and thus it is preferably used as the materialof the sacrificial layer. In the etching step executed in the subsequentstep, the etching rate of about 3 μm per minute in the dilute ratio of10:1 is achieved.

The surface of the PSG sacrificial layer 55 would be very rough at theatomic level if it is left as it is. Accordingly, the PSG sacrificiallayer 55 as deposited is insufficient as a base body to form the thinfilm bulk acoustic resonator. The FBAR/SBAR type thin film bulk acousticresonator needs a piezoelectric material whose crystal is grown asprismatic or columnar crystal vertical to the plane of the electrode. Bypolishing and smoothening the surface of the PSG sacrificial layer 55with polishing slurry containing fine polishing particles, the lowerelectrode layer having excellent orientation and crystal quality can beformed, and thus the piezoelectric thin film having excellentorientation and crystal quality can be formed.

That is, as shown in FIG. 18, the surface of the PSG sacrificial layer55 is polished with roughly finishing slurry to be flattened orplanarized, and a portion of the PSG layer deposited on the adhesionelectrode layer 161 is removed. Subsequently, the remaining PSG layer 55may be further polished by using precisely finishing slurry containingfiner polishing particles. As a substitute method, one finer preciselyfinishing slurry may be used in the two polishing steps if furtherpolishing time is permitted. The target is to implement finishing like“mirror” (mirror-like finishing).

In the present invention, before the PSG layer is polished, it ispreferable to subject the PSG layer to heat treatment at hightemperature for the purpose of densification and reflow. The heattreatment of the PSG layer may be performed by RTA (Rapid ThermalAnneal) method. This is carried out at a temperature range from 750° C.to 950° C. under nitrogen atmosphere or nitrogen-oxygen mixtureatmosphere. Alternatively, the high-temperature heat treatment may becarried out in a diffusion furnace or by lamp heating. In the presentinvention, by subjecting the PSG layer to the high-temperature heattreatment, the PSG layer can be designed to have a more densifiedstructure and the hardness thereof is increased. By increasing thehardness of the PSG layer, polishing damage such as scratch or the likeis prevented from occurring on the surface of the PSG film in thesubsequent CMP (Chemical Mechanical Polishing), and the surface of thePSG film can be excellently flattened.

It is important to clean the substrate after the polishing as describedabove. The slurry causes a slight amount of silica coarse grain toremain on the substrate. Accordingly, the coarse grain must be removed.In a preferred embodiment of the present invention, the removal of thecoarse grain is carried out by using a second polishing tool having arigid pad such as Polytex (trademark) produced by Rodel NittaIncorporated Company. In this case, deionized water is used aslubricant, and the wafer is immersed in the deionized water from thetime when the substrate has been polished until the time when thestandby of the final cleaning step is completed. It should be noted thatthe substrate is prevented from being dried from the last polishing stepto the final cleaning step. The final cleaning step comprises a step ofimmersing the substrate into a series of tanks which are charged withvarious chemicals. Ultrasonic stirring is applied to each tank. Suchcleaning means is known to persons skilled in the art.

The polishing material is formed of silica fine particles. In thepreferred embodiment of the present invention, ammonia-based slurry ofsilica fine particle (Rodel Klebosol#30N: Rodel Nitta IncorporatedCompany) is used as the polishing material.

In the foregoing description, the specific polishing and cleaningmanners are described. However, any polishing and cleaning manners maybe used insofar as they can give required smooth surface. In thepreferred embodiment of the present invention, with respect to thefinished surface, the RMS variation of the height of the surface whichis measured by an atomic force microscope probe is equal to 25 nm orless, preferably 20 nm or less, and more preferably 10 nm or less insurface roughness.

After the surface is smoothened and further the surface of the adhesionelectrode layer 161 is cleaned by plasma etching, the lower electrodelayer 162 of the sandwich structure 60 is deposited as shown in FIG. 19.Au, Pt, W or Mo is preferably used as the material of the lowerelectrode layer 162. The orientation and crystal quality of thepiezoelectric thin film layer 163 formed on the lower electrode layer162 reflect the orientation and crystallinity of the lower electrodelayer 162.

The thickness of the lower electrode layer 162 is important. The thicklayer has rougher surface than the thin layer. It is very important forthe performance of the resonator achieved that the smooth surface iskept for deposition of the piezoelectric thin film layer as describedabove. Accordingly, the thickness of the lower electrode layer 162 ispreferably less than 200 nm. Au, Pt, W or Mo is preferably deposited bysputtering. According to this method, there is achieved the lowerelectrode layer 162 having the RMS variation of the height of thesurface that is equal to 25 nm or less, preferably 20 nm or less, morepreferably 10 nm or less in surface roughness.

After the deposition of the lower electrode layer 162 is finished, thePSG sacrificial layer remaining around the lower electrode layer 162 isremoved, and the piezoelectric thin film layer 163 is deposited. AlN orZnO is preferably used as the material of the piezoelectric thin filmlayer 163, and it is deposited by sputtering. In the preferredembodiment of the present invention, the thickness of the piezoelectricthin film layer 163 is set to a value in the range from 0.1 μm to 10 μm,preferably to a value in the range from 0.5 μm to 2 μm.

Finally, the upper electrode layer 164 is deposited. The upper electrodelayer 164 is formed of the same material as the lower electrode layer162, and Au, Pt, W or Mo is preferably used.

After the sandwich structure 60 which comprises the joined structure ofthe adhesion electrode layer 161, the lower electrode layer 162, thepiezoelectric thin film layer 163 and the upper electrode layer 164 andis patterned to have a desired shape is formed as described above, asmall through hole extending downwardly from the peripheral portion ofthe upper electrode layer 164 through the upper electrode layer 164, thepiezoelectric thin film layer 163 and the lower electrode layer 162 tothe sacrificial layer 55 is formed by a dry etching method such as RIE(reactive ion etching) or the like, and then etching using dilute H₂O:HFsolution is carried out to remove PSG below the sandwich structure 60,whereby the bridged sandwich structure 60 remains on the recess 52 asshown in FIGS. 20 and 21. That is, the sandwich structure 60 is disposedso that the adhesion electrode layer 161 is located around the pit 52formed on the surface of the substrate and the edge portion of thesandwich structure 60 is supported on the substrate so as to stride overthe pit 52.

In the thin film bulk acoustic resonator thus achieved, the weight isincreased at the peripheral portion of the sandwich structure 60 by theamount corresponding to the adhesion electrode layer 161, so thatoccurrence of vibration in the lateral direction can be suppressed andextra spurious vibration can be prevented from being superposed on thevibration in the thin film bulk acoustic resonator. The formation of theadhesion electrode layer 161 around the pit 52 makes it possible todeposit the lower electrode layer of Au, Pt or the like which has notbeen hitherto able to solely deposit on the cavity, and the adhesionbetween the lower electrode layer of W, Mo or the like and the basesubstrate can be improved.

According to the manufacturing method of the thin film bulk acousticresonator as described above, the center portion of the lower electrodelayer 162 of Au, Pt, W, Mo or the like is formed on glassy sacrificiallayer such as silica glass, phospho-silicate glass or the like, and thusthe orientation and crystallinity of the lower electrode layer is moreexcellent than the conventional case where the electrode layer of Au, PtW, Mo or the like is wholly formed on the adhesion layer of Ti or thelike, so that excellent crystal film having a small full width at halfmaximum (FWHM) of diffraction peak in the rocking curve can be achieved.By improving the orientation and crystal quality of the lower electrodelayer 162 as described above, the orientation and crystal quality of thepiezoelectric thin film layer formed on the lower electrode layer 162can be improved.

The above-described embodiment relates to FBAR. However, it is clearthat persons skilled in the art can manufacture SBAR by using the sameprocess from the foregoing description. In the case of SBAR, anotherpiezoelectric layer (second piezoelectric layer) and an electrode layeron the piezoelectric layer must be deposited. The second piezoelectriclayer is formed on the upper electrode layer of “FBAR” shown in theabove-described embodiment. Therefore, the thickness of the upperelectrode layer is kept to, for example, 100 nm so that such a propersurface state that the second piezoelectric layer can be deposited onthe surface can be given. For example, it is preferable that the surfaceis smoothened so that the RMS variation of the height thereof is equalto 25 nm or less, preferably to 20 nm or less and more preferably to 10nm or less in surface roughness.

In the above-described embodiment of the present invention, thesacrificial layer formed of PSG is used, however, other materials may beused for the sacrificial layer. For example, BPSG(Boron-Phosphor-silicate-Glass) or other type glass like spin glass maybe used. Besides, plastics such as polyvinyl, polypropylene,polystyrene, etc. which can be deposited on the substrate by spinningmay be used. When the sacrificial layer is formed of each of thesematerials, it is important to smoothen the surface by polishing as inthe case of the PSG sacrificial layer. These sacrificial layers may beremoved by organic removing agent or O₂ plasma etching.

Next, FIG. 22 is a schematic plan view showing an embodiment of thepiezoelectric thin film resonator (thin film bulk acoustic resonator)according to the present invention, and FIG. 23 is a cross-sectionalview taken along X—X of FIG. 22. In FIGS. 22 and 23, a piezoelectricthin film resonator 111 has a substrate 112, an insulating layer 13formed on the upper surface of the substrate 112, and a piezoelectricstack structure 14 joined onto the upper surface of the insulating layer13.

The piezoelectric stack structure 14 comprises a lower electrode 15formed on the upper surface of the insulating layer 13, a piezoelectricfilm 16 formed on the upper surface of the base film 13 so as to cover apart of the lower electrode 15 and an upper electrode 17 formed on theupper surface of the piezoelectric film 16. A via hole 120 forming acavity is formed in the substrate 112. A part of the insulating layer 13is exposed to the via hole 120. The exposed portion of the insulatinglayer 13 and the portion of the piezoelectric stack structure 14 whichcorresponds to the exposed portion concerned constitute a vibratingportion (vibrating diaphragm) 121. The lower electrode 15 and the upperelectrode 17 have main body portions 15 a, 17 a formed in the areacorresponding to the vibrating portion 121 and terminal portions 15 b,17 b for connecting the main body portions 15 a, 17 a to an externalcircuit. The terminal portions 15 b, 17 b are located out of the areacorresponding to the vibrating portion 121.

As the substrate 112 may be used single crystal such as Si(100) singlecrystal or the like, or base material such as Si single crystal or thelike on which polycrystalline film of silicon, diamond or the like isformed. An anisotropic etching method of starting anisotropic etchingfrom the lower surface side of the substrate may be used as the methodof forming the via hole 120 of the substrate 112. The forming source forthe cavity to be formed in the substrate 112 is not limited to the viahole 120, and any source may be used insofar as it permits vibration ofthe vibrating portion 121. For example, a pit portion or cavity portionor recess portion formed in an area of the upper surface of thesubstrate which corresponds to the vibrating portion 121 may be used.

The insulating layer 13 is formed of dielectric film mainly comprisingsilicon oxide (SiO₂) or silicon nitride (SiN_(x)) (preferably thecontent thereof is equal to 50 equivalent % or more). The dielectricfilm may comprise a monolayer or plural layers containing a layer forincreasing adhesion force, etc. As an example of the dielectric filmcomprising plural layers, a silicon nitride layer added onto one surfaceor both the surfaces of an SiO₂ layer may be used. The thickness of theinsulating layer 13 is equal to 0.2 to 2.0 μm, for example. As themethod of forming the insulating layer 13 may be used the thermaloxidation method of the surface of the substrate 112 of silicon, the CVDmethod or the low-pressure CVD method.

Each of the lower electrode 15 and the upper electrode 17 comprises aconductive film mainly formed of molybdenum (Mo) (preferably, thecontent thereof is 80 atomic % or more). Since Mo has a lowthermoelastic loss (about {fraction (1/56)} of Al), it is preferablyused particularly to construct a vibrating portion vibrating at highfrequencies. Not only Mo is used alone, but also alloy mainly containingMo may be used. The thickness of each of the lower electrode 15 and theupper electrode 17 is equal to 50 to 200 nm, for example. A sputteringmethod or deposition method may be used to form the lower electrode 15and the upper electrode 17. As occasion demands, a photolithographytechnique is used for patterning to achieve a desired shape.

The piezoelectric film 16 comprises a piezoelectric film mainly formedof AlN (preferably, the content thereof is 90 equivalent % or more), andthe thickness thereof is equal to 0.5 to 2.5 μm, for example. A reactivesputtering method may be used to form the piezoelectric film 16, and asoccasion demands, the photolithography technique is used for patterningto achieve a desired shape.

With respect to FBAR that has the piezoelectric film 16 having theconstruction shown in FIGS. 22 and 23 and mainly formed of AlN and alsohaving a fundamental mode in the neighborhood of 2 GHz, the inventorshave dedicated to improving the temperature stability of the resonancefrequency for FBAR without degrading the electromechanical couplingcoefficient and the acoustic quality factor while taking advantage ofthe feature of the AlN thin film that the propagation velocity ofelastic wave is high, and as a result they have discovered that it iseffective to use an insulating layer 13 mainly formed of SiO₂ or SiN_(x)and also use upper and lower electrodes 15, 17 mainly formed of Mo.Further, they have discovered that all of the electromechanical couplingcoefficient, the acoustic quality factor and the temperature stabilityof the resonance frequency are furthermore greatly improved bysatisfying the following inequality: 0.1≦t′/t≦0.5, preferably0.2≦t′/t≦0.4, wherein t represents the thickness of the piezoelectricfilm 16 and t′ represents the thickness of the insulating layer 13. Ift′/t<0.1, the electromechanical coupling coefficient and the acousticquality factor may be slightly increased in some cases, however, thereis a tendency that the absolute value of the temperature coefficient ofthe resonance frequency is increased and the characteristic as FBAR islowered. On the other hand, if t′/t>0.5, there is a tendency that theelectromechanical coupling coefficient and the acoustic quality factorare lowered and the absolute value of the temperature coefficient of theresonance frequency is increased, so that the characteristic as FBAR islowered.

FIG. 24 is a schematic plan view showing another embodiment of thepiezoelectric thin film resonator according to the present invention,and FIG. 25 is a cross-sectional view taken along X—X of FIG. 24. InFIGS. 24 and 25, the elements having the same functions as those ofFIGS. 22 and 23 are represented by the same reference numerals.

In this embodiment, in addition to the insulating layer 13, aninsulating layer 18 mainly formed of SiO₂ or SiN_(x) (preferably thecontent thereof is equal to 50 equivalent % or more) is joined to thepiezoelectric stack structure 14. The insulating layer 18 is formed onthe main body portion 17 a of the upper electrode 17. The insulatinglayer 18 may be formed in a broad area on the piezoelectric film 16 soas to extend further outwardly from the area corresponding to thevibrating portion 121. Further, when the insulating layer 18 mainlyformed of silicon oxide or silicon nitride is formed, the insulatinglayer 13 may be omitted. However, in this case, it is preferable thatthe main body portion 15 a of the lower electrode 15 extends through twosides of a rectangular opening of the via hole 120 at the upper surfaceof the substrate 112 to the inside of the opening so that the vibratingportion 121 is held by the lower electrode 15.

The embodiment shown in FIGS. 24 and 25 has the same effect as that ofFIGS. 22 and 23.

FIG. 26 is a schematic plan view showing another embodiment of thepiezoelectric thin film resonator according to the present invention,and FIG. 27 is a cross-sectional view taken along X—X of FIG. 26. InFIGS. 26 and 27, the same elements having the same function as those ofFIGS. 22 to 25 are represented by the same references.

In this embodiment, the lower electrode 15 has a rectangular shape, andthe upper electrode 17 comprises a first electrode portion 17A and asecond electrode portion 17B. The electrode portions 17A, 17B have mainbody portions 17Aa, 17Ba and terminal portions 17Ab, 17Bb, respectively.Each of the main body portions 17Aa, 17Ba is located in the areacorresponding to the vibrating portion 121, and each of the terminalportions 17Ab, 17Bb is located out of the area corresponding to thevibrating portion 121.

FIG. 28 is a schematic plan view showing another embodiment of thepiezoelectric thin film resonator according to the present invention,and FIG. 29 is a cross-sectional view taken along X—X of FIG. 28. InFIGS. 28 and 29, the elements having the same functions as those ofFIGS. 22 to 27 are represented by the same reference numerals.

In this embodiment, the lower electrode 15 has a rectangular shape, andthe upper electrode 17 comprises a first electrode portion 17A and asecond electrode portion 17B. These electrode portions 17A, 17B havemain body portions 17Aa, 17Ba and terminal portions 17Ab, 17Bb,respectively. Each of the main body portions 17Aa, 17Ba is located inthe area corresponding to the vibrating portion 121, and each of theterminal portions 17Ab, 17Bb is located out of the area corresponding tothe vibrating portion 121. In this embodiment, the insulating layer 18is formed so as to cover both the main body portion 17Aa of the firstelectrode portion and the main body portion 17Ba of the second electrodeportion.

The embodiment shown in FIGS. 26, 27 and the embodiment of FIGS. 28, 29have the same effect as the embodiment of FIGS. 22, 23 and theembodiment of FIGS. 24, 25. The embodiments shown in FIGS. 26, 27 andFIGS. 28, 29 relate to a so-called multiple-mode resonator. An inputvoltage is applied between one of the upper electrodes 17 (for example,the second electrode portion 17B) and the lower electrode 15, and thevoltage between the other of the upper electrodes 17 (for example, thefirst electrode portion 17A) and the lower electrode 15 can be taken outas an output voltage.

In the piezoelectric thin film resonator as described above, thefollowing relationship (equations) is satisfied among theelectromechanical coupling coefficient k_(t) ² and the resonantfrequency f_(r) and antiresonant frequency f_(a) in the impedancecharacteristic measured by using a microwave prober:k _(t) ²=φ_(r)/Tan(φ_(r))φ_(r)=(π/2)(f _(r) /f _(a))

For the purpose of simple description, the electromechanical couplingcoefficient k_(t) ² may be calculated by using the following equation:k _(t) ²=4.8(f _(a) −f _(r))/(f _(a) +f _(r))In this specification, values of the electromechanical couplingcoefficient k_(t) ² calculated by the above equation are used.

FBARs having the constructions shown in FIGS. 22 to 29, theelectromechanical coupling coefficients obtained from the measurementvalues of the resonant frequency and the antiresonant frequency in theneighborhood of 2.0 GHz are equal to 4.0 to 6.5%. If theelectromechanical coupling coefficient k_(t) ² is less than 4.0%, therewould be a tendency that the band width of FBAR manufactured is reducedand thus it is difficult to practically use FBAR in a high-frequencyband area.

The present invention will be described in more detail by the followingExamples.

EXAMPLE 1

A thin film bulk acoustic resonator was manufactured in the manner asshown in FIGS. 3 to 8.

First, the surface of a Si wafer 51 was coated with a Pt/Ti protectionfilm, and the protection film was etched to have a predetermined patternfor formation of a pit or cavity or recess, thereby forming a mask foretching the Si wafer 51. Thereafter, wet etching was carried out byusing the Pt/Ti pattern mask thus formed to form a pit or cavity orrecess of 20 μm in depth and 150 μm in width. The etching was carriedout with KOH water solution of 5% by weight at a liquid temperature of70° C. Alternatively, the depth of the pit may be set to 3 μm.

Thereafter, the Pt/Ti pattern mask was removed, and a SiO₂ layer 53 of 1μm in thickness was formed on the surface of the Si wafer 51 by thermaloxidation, thereby achieving such a structure that the pit 52 is formedin the substrate comprising the Si wafer 51 and the SiO₂ layer 53.

Subsequently, as shown in FIG. 4, a PSG sacrificial layer 55 of 30 μm inthickness was deposited on the SiO₂ layer having the pit 52 formedthereon. This deposition was carried out at 450° C. by the thermal CVDmethod using silane and P₂O₅ as raw materials. Alternatively, thethickness of the PSG sacrificial layer 55 may be set to 5 μm, and silaneand trimethyl phosphate (PO(OCH₃)₃) may be used as raw materials in thethermal CVD method. Further, the PSG sacrificial layer deposited may besubjected to a heat treatment at 850° C. for 20 minutes under the 1%oxygen/nitrogen mixture atmosphere and reflowed to increase the hardnessof the PSG sacrificial layer.

Subsequently, as shown in FIG. 5, the surface of the PSG sacrificiallayer 55 was polished to remove the PSG sacrificial layer 55 in the areaout of the pit 52. Subsequently, the surface of the PSG sacrificiallayer 55 remaining in the pit 52 was polished with slurry containingfine polishing particles to set the RMS variation of the height of thesurface to 10 nm.

Subsequently, as shown in FIG. 6, a lower electrode 61 comprising a Mofilm of 100 nm in thickness and 200×200 μm in size was formed on the PSGsacrificial layer 55. The formation of the Mo film was carried out atroom temperature by a DC magnetron sputtering method using Ar as sputtergas. The Mo film was subjected to a patterning treatment by a lift-offmethod. The surface roughness of the Mo film thus formed was measured,and the result indicated that the RMS variation of the height was equalto 15 nm.

Subsequently, a piezoelectric layer 62 comprising a ZnO film of 1.0 μmin thickness was formed on the lower electrode 61. The formation of theZnO film was carried out by using the RF magnetron sputtering methodunder the condition that ZnO was used as sputtering target, mixture gasof Ar and O₂ was used as sputter gas, the sputter gas pressure was setto 5 mTorr and the substrate temperature was set to 400° C. The surfaceroughness of the ZnO film thus formed was measured, and the resultindicated that the RMS variation of the height was equal to 11 nm, whichwas not more than 5% of the film thickness. The ZnO film was patternedinto a predetermined shape by wet etching to achieve the piezoelectriclayer 62.

Subsequently, an upper electrode comprising a Mo film of 100 nm inthickness was formed on the piezoelectric layer 62. The formation andpatterning of the Mo film were carried out in the same manner as theformation of the lower electrode 61. With respect to the surface of theupper electrode 63, the waviness height was measured at the measurementlength of 150 μm, and the result was equal to 0.2 μm which was not morethan 25% of the thickness of the piezoelectric layer 62 and also notmore than 0.5% of the measurement length.

Subsequently, the PSG sacrificial layer 55 was removed by the etchingusing dilute H₂O:HF solution, thereby forming such a structure that thesandwich structure 60 of Mo/ZnO/Mo was suspended over the pit 52 asshown in FIG. 8.

Thin film XRD analysis was conducted on the piezoelectric layer 62 thusachieved. The result was that the c-axis of the film was inclined at anangle of 88.5 degrees with respect to the film surface. Further, theorientation was examined on the basis of the rocking curve, and theresult was that the full width at half maximum of the peak was equal to2.5 degrees and excellent orientation was exhibited.

With respect to the acoustic resonator thus achieved, the impedancecharacteristic between the upper electrode 63 and the lower electrode 61was measured by using a microwave prober, the resonant frequency f_(r)and the antiresonant frequency f_(a) were measured, and then theelectromechanical coupling coefficient k_(t) ² was calculated on thebasis of these measurement values. The electromechanical couplingcoefficient k_(t) ² was equal to 5.5%, and the acoustic quality factorwas equal to 700. The construction of FBAR achieved in EXAMPLE 1 and thecharacteristics thereof as the acoustic resonator are shown in Table 1.

COMPARATIVE EXAMPLE 1

An acoustic resonator was manufactured in the same manner as EXAMPLE 1except that the surface of the PSG sacrificial layer 55 was polished sothat the RMS variation of the height thereof (surface roughness) wasequal to 70 nm.

The surface roughness of the Mo film of the lower electrode 61 wasmeasured, and the result was that the RMS variation of the height wasequal to 80 nm. Further, the surface roughness of the ZnO film wasmeasured, and the result was that the RMS variation of the height wasequal to 75 nm (exceeding 5% of the film thickness). With respect to thesurface of the upper electrode 63, the waviness height at themeasurement length of 150 μm was measured, and the result was that itwas equal to 1.0 μm (exceeding 0.5% of the measurement length).

Further, the thin film XRD analysis was conducted on the piezoelectriclayer 62 thus achieved, so that the c-axis of the film was greatlyinclined at an angle of 85.0 degrees with respect to the film surface.The orientation was examined on the basis of the rocking curve, so thatthe full width at half maximum of the peak was equal to 7.0 degrees.

The electromechanical coupling coefficient k_(t) ² of the acousticresonator thus achieved was equal to 3.0%, and the acoustic qualityfactor thereof was equal to 400. The construction of FBAR achieved inCOMPARATIVE EXAMPLE 1 and the characteristics thereof as the acousticresonator are shown in Table 1.

EXAMPLE 2

An acoustic resonator was manufactured in the same manner as EXAMPLE 1except that AlN film was used as the piezoelectric film 62 in place ofthe ZnO film. That is, the piezoelectric layer 62 comprising an AlN filmof 1.2 μm in thickness was formed on the lower electrode 61. Theformation of the AlN film was carried out by using the RF magnetronsputtering method at a substrate temperature of 400° C. under thecondition that Al was used as sputtering target and the mixture gas ofAr and N₂ was used as sputter gas. The surface roughness of the AlN filmthus formed was measured, and the result was that the RMS variation ofthe height was equal to 14 nm which was not more than 5% of the filmthickness. With respect to the surface of the upper electrode 63, thewaviness height at the measurement length of 150 μm was measured, sothat the it was equal to 0.2 μm which was not more than 25% of thethickness of the piezoelectric layer 62 and also not more than 0.5% ofthe measurement length.

Further, the thin film XRD analysis was conducted on the piezoelectriclayer 62 thus achieved, so that the c-axis of the film was inclined atan angle of 88.5 degrees with respect to the film surface. Further, theorientation was examined on the basis of the rocking curve, so that thefull width at half maximum of the peak was equal to 2.8 degrees andexcellent orientation was exhibited.

The electromechanical coupling coefficient k_(t) ² of the acousticresonator thus achieved was equal to 6.5%, and the acoustic qualityfactor was equal to 900. The construction of FBAR achieved in EXAMPLE 2and the characteristics thereof as the acoustic resonator are shown inTable 1.

COMPARATIVE EXAMPLE 2

An acoustic resonator was manufactured in the same manner as EXAMPLE 2except that the surface of the PSG sacrificial layer 55 was polished sothat the RMS variation of the height thereof (surface roughness) wasequal to 70 nm.

The surface roughness of the Mo film of the lower electrode 61 wasmeasured, and the result was that the RMS variation of the height wasequal to 85 nm. Further, the surface roughness of the AlN film wasmeasured, and the result was that the RMS variation of the height wasequal to 80 nm (exceeding 5% of the film thickness). With respect to thesurface of the upper electrode 63, the waviness height at themeasurement length of 150 μm was investigated, so that it was equal to1.25μm (exceeding 0.5% of the measurement length).

Further, the thin film XRD analysis was conducted on the piezoelectriclayer 62 thus achieved, so that the c-axis of the film was greatlyinclined at an angle of 83.0 degrees with respect to the film surface.The orientation was examined on the basis of the rocking curve, so thatthe full width at half maximum of the peak was equal to 8.5 degrees.

The electromechanical coupling coefficient k_(t) ² of the acousticresonator thus achieved was equal to 3.5%, and the acoustic qualityfactor thereof was equal to 450. The construction of FBAR achieved inCOMPARATIVE EXAMPLE 2 and the characteristics thereof as the acousticresonator are shown in Table 1.

EXAMPLE 3

A thin film bulk acoustic resonator was manufactured in the same manneras shown in FIGS. 3 to 5 and FIGS. 9 to 10.

First, a structure shown in FIG. 5 was achieved in the same manner asEXAMPLE 1. However, the surface of the PSG sacrificial layer 55remaining in the pit 52 was polished with slurry containing finepolishing particles so that the RMS variation of the height of thesurface was equal to 5 nm.

Subsequently, as shown in FIG. 9, the insulating layer 54 comprising aSiO₂ film of 500 nm in thickness was formed on the substrate by the CVDmethod so that the surface of the PSG sacrificial layer 55 was alsocovered by the insulating layer 54. The surface roughness of theinsulating layer 54 thus formed was measured, so that the RMS variationof the height was equal to 10 nm.

Subsequently, as in the case of EXAMPLE 1, the lower electrode 61comprising a Mo film was formed on the insulating layer 54 as shown inFIG. 10. The surface roughness of the Mo film thus formed was measured,so that the RMS variation of the height was equal to 15 nm.

Subsequently, the piezoelectric layer 62 comprising a ZnO film wasformed on the lower electrode 61 in the same manner as EXAMPLE 1. Thesurface roughness of the ZnO film thus formed was measured, so that theRMS variation of the height was equal to 10 nm (not more than 5% of thefilm thickness). The ZnO film was patterned into a predetermined shapeby wet etching to achieve the piezoelectric layer 62.

Subsequently, the upper electrode 63 comprising a Mo film was formed onthe piezoelectric layer 62 in the same manner as EXAMPLE 1. With respectto the surface of the upper electrode 63, the waviness height at themeasurement length 150 μm was investigated, so that it was equal to 0.2μm (not more than 25% of the thickness of the piezoelectric layer 62 andalso not more than 0.5% of the measurement length).

Subsequently, a via hole was formed at the exposed portion of theinsulating layer 54 so as to reach the PSG sacrificial layer 55, andetching was carried out through the via hole with dilute H₂O:HF solutionto remove the PSG sacrificial layer 55, thereby forming a structure thatthe stack structure having the insulating layer 54 and the sandwichstructure 60 of Mo/ZnO/Mo was suspended over the pit 52.

The thin film XRD analysis was conducted on the piezoelectric layer 62thus achieved, so that the c-axis of the film was inclined at an angleof 88.5 degrees with respect to the film surface. Further, theorientation was investigated on the basis of the rocking curve, so thatthe full width at half maximum of the peak was equal to 2.3 degrees andexcellent orientation was exhibited.

With respect to the acoustic resonator thus achieved, the impedancecharacteristic between the upper electrode 63 and the lower electrode 61was measured by using a microwave prober, the resonant frequency f_(r)and the antiresonant frequency f_(a) were measured, and theelectromechanical coupling coefficient k_(t) ² was calculated on thebasis of these measurement values. The electromechanical couplingcoefficient k_(t) ² was equal to 4.5%, and the acoustic quality factorwas equal to 650. The construction of FBAR achieved in EXAMPLE 3 and thecharacteristics thereof as the acoustic resonator are shown in Table 1.

COMPARATIVE EXAMPLE 3

An acoustic resonator was manufactured in the same manner as EXAMPLE 3except that the surface of the PSG sacrificial layer 55 was polished sothat the RMS variation of the height was equal to 70 nm.

The surface roughness of the SiO₂ film of the insulating layer 54 wasmeasured, so that the RMS variation of the height was equal to 85 nm.The surface roughness of the Mo film of the lower electrode 61 wasmeasured, so that the RMS variation of the height was equal to 90 nm.The surface roughness of the ZnO film was measured, so that the RMSvariation of the height was equal to 85 nm (exceeding 5% of the filmthickness). With respect to the surface of the upper electrode 63, thewaviness height at the measurement length of 150 μm was examined, sothat it was equal to 1.0 μm (exceeding 0.5% of the measurement length).

The thin film XRD analysis was conducted on the piezoelectric layer 62thus achieved, so that the c-axis of the film was greatly inclined at anangle of 83.0 degrees with respect to the film surface. Further, theorientation was investigated on the basis of the rocking curve, so thatthe full width at half maximum of the peak was equal to 9.5 degrees.

The electromechanical coupling coefficient k_(t) ² of the acousticresonator thus achieved was equal to 2.8%, and the acoustic qualityfactor was equal to 360. The construction of FBAR achieved inCOMPARATIVE EXAMPLE 3 and the characteristics thereof as the acousticresonator are shown in Table 1.

EXAMPLE 4

An acoustic resonator was manufactured in the same manner as EXAMPLE 2with the exception of a manner of forming the upper electrode 63 Thatis, after the Mo film of 100 nm in thickness was formed on thepiezoelectric layer 62 in the same manner as EXAMPLE 2, a Mo film of 20nm in thickness was formed on the 100 nm-thick Mo film in an areaextending inward from the outer edge thereof by a distance of 30 μm(i.e., the width of the area is equal to 30 μm) by the lift-off method,thereby forming the upper electrode 63 as shown in FIG. 11.

With respect to the surface of the center portion 631 of the upperelectrode 63, the waviness height at the measurement length of 100 μmwas investigated, so that it was equal to 0.15 μm (not more than 25% ofthe film thickness of the piezoelectric layer 62 and not more than 0.5%of the measurement length).

The electromechanical coupling coefficient k_(t) ² of the acousticresonator thus achieved was equal to 7.5%, and the acoustic qualityfactor was equal to 950. The construction of FBAR achieved in EXAMPLE 4and the characteristics thereof as the acoustic resonator are shown inTable 1.

EXAMPLE 5

A thin film bulk acoustic resonator was manufactured in the same manneras shown in FIGS. 3 to 8.

First, the surface of a Si wafer 51 was coated with a SiO₂ protectionfilm, and the protection film was etched to have a predetermined patternfor formation of a recess, thereby forming a mask for etching the Siwafer 51. Thereafter, wet etching was carried out by using the mask toform a pit or cavity or recess of 3 μm in depth and 150 μm in width asshown in FIG. 3. This etching was carried out in the same manner asEXAMPLE 1.

Thereafter, the SiO₂ pattern mask was removed by the wet etching, and aSi₃N₄ layer 53 of 200 nm in thickness was formed on the surface of theSi wafer 51 as shown in FIG. 3, thereby achieving a structure that thepit 52 was formed on the substrate comprising the Si wafer 51 and theSi₃N₄ layer 53. The deposition of the Si₃N₄ layer 53 was carried out at800° C. by the low-pressure CVD method using monosilane (SiH₄) andammonia (NH₃) as raw materials.

Subsequently, as shown in FIG. 4, a PSG sacrificial layer 55 of 5 μm inthickness was deposited on the Si₃N₄ layer 53 having the pit 52 formedtherein. This deposition was carried out at 450° C. by the thermal CVDmethod using tetraethoxysilane or silicon tetraethoxide (Si(OC₂H₅)₄) andtrimethyl phosphate (PO(OCH₃)₃) as raw materials. Further, the PSGsacrificial layer thus deposited was subjected to heat treatment at 850°C. for 20 minutes under the 1% oxygen/nitrogen mixture atmosphere to bereflowed, so that the hardness of the PSG sacrificial layer wasincreased.

Subsequently, the structure shown in FIG. 5 was achieved in the samemanner as EXAMPLE 1. Proper polishing particles were selected so thatthe RMS variation of the height of the surface of the PSG sacrificiallayer 55 remaining in the recess 52 was equal to 5 nm.

Subsequently, the lower electrode 61 comprising a Mo film was formed asshown in FIG. 6 in the same manner as EXAMPLE 1. The surface roughnessof the Mo film thus formed was measured, so that the RMS variation ofthe height was equal to 13 nm.

Subsequently, a piezoelectric layer 62 comprising an AlN film of 1.2 μmin thickness was formed on the lower electrode 61. The surface roughnessof the AlN film thus formed was measured, so that the RMS variation ofthe height was equal to 10 nm (not more than 5% of the film thickness).

Subsequently, the upper electrode 63 comprising a Mo film was formed onthe piezoelectric layer 62 in the same manner as EXAMPLE 1. With respectto the surface of the upper electrode 63, the waviness height at themeasurement length of 150 μm was examined, so that it was equal to 0.15μm (not more than 25% of the thickness of the piezoelectric layer 62 andnot more than 0.5% of the measurement length).

Subsequently, in the same manner as EXAMPLE 1, the PSG sacrificial layer55 was removed, thereby forming a structure that the sandwich structure60 of Mo/AlN/Mo was suspended over the pit 52 as shown in FIG. 8.

The thin film XRD analysis was conducted on the piezoelectric layer 62thus achieved, so that the c-axis of the film was inclined at an angleof 89.5 degrees with respect to the film surface. The orientation wasexamined on the basis of the rocking curve, so that the full width athalf maximum (FWHM) of the peak was equal to 2.2 degrees and excellentorientation was exhibited.

With respect to the acoustic resonator thus achieved, the impedancecharacteristic between the upper electrode 63 and the lower electrode 61was measured by using a microwave prober, the resonant frequency f_(r)and the antiresonant frequency f_(a) were measured, and theelectromechanical coupling coefficient k_(t) ² was calculated on thebasis of these measurement values. The electromechanical couplingcoefficient k_(t) ² was equal to 6.7%, and the acoustic quality factorwas equal to 980. The construction of FBAR achieved in EXAMPLE 5 and thecharacteristics thereof as the acoustic resonator are shown in Table 1.

EXAMPLE 6

A thin film bulk acoustic resonator was manufactured in the same manneras shown in FIGS. 3 to 5 and FIGS. 9 and 10.

First, a structure shown in FIG. 5 was achieved in the same manner asEXAMPLE 5. However, by selecting proper polishing particles, the surfaceroughness of the PSG sacrificial layer 55 remaining in the pit 52 wasset so that the RMS variation of the height was equal to 10 nm.

Subsequently, as shown in FIG. 9, the insulating layer 54 comprisingSi₃N₄ film of 500 nm in thickness was formed on the substrate so thatthe surface of the PSG sacrificial layer 55 was also covered by theinsulating layer 54. The deposition of the insulating layer 54comprising the Si₃N₄ film was carried out at 800° C. by using thelow-pressure CVD method using monosilane (SiH₄) and ammonia (NH₃) as rawmaterials. The surface roughness of the insulating layer 54 thus formedwas measured, so that the RMS variation of the height was equal to 12nm.

Subsequently, the lower electrode 61 comprising a Mo film was formed onthe insulating layer 54 in the same manner as EXAMPLE 5 as shown in FIG.10. The surface roughness of the Mo film thus formed was measured, sothat the RMS variation of the height was equal to 17 nm.

Subsequently, the piezoelectric layer 62 comprising an AlN film wasformed on the lower electrode 61. The surface roughness of the AlN filmthus formed was measured, so that the RMS variation of the height wasequal to 15 nm (not more than 5% of the film thickness).

Subsequently, the upper electrode 63 comprising a Mo film was formed onthe piezoelectric layer 62 in the same manner as EXAMPLE 5. With respectto the surface of the upper electrode 63, the waviness height at themeasurement length of 150 μm was examined, so that it was equal to 0.21μm (not more than 25% of the thickness of the piezoelectric layer 62 andnot more than 0.5% of the measurement length).

Subsequently, in the same manner as EXAMPLE 3, the PSG sacrificial layer55 was removed, thereby forming a structure that the stack structure ofthe insulating layer 54 and the sandwich structure 60 of Mo/AlN/Mo aresuspended over the pit 52 as shown in FIG. 10.

The thin film XRD analysis was conducted on the piezoelectric layer 62thus achieved, so that the c-axis of the film was inclined at an angleof 88.4 degrees with respect to the film surface. Further, theorientation was examined on the basis of the rocking curve, so that thefull width at half maximum (FWHM) of the peak was equal to 2.8 degreesand excellent orientation was exhibited.

With respect to the acoustic resonator thus achieved, the impedancecharacteristic between the upper electrode 63 and the lower electrode 61was measured by using a microwave prober, the resonant frequency f_(r)and the antiresonant frequency f_(a) were measured, and theelectromechanical coupling coefficient k_(t) ² was calculated on thebasis of these measurement values. The electromechanical couplingcoefficient k_(t) ² was equal to 5.2%, and the acoustic quality factorwas equal to 700. The construction of FBAR achieved in EXAMPLE 6 and thecharacteristics thereof as the acoustic resonator are shown in Table 1.

EXAMPLE 7

A thin film bulk acoustic resonator was manufactured in the manner asshown in FIGS. 15 to 21.

First, the surface of a Si wafer 51 was coated with a SiO₂ protectionfilm, and the protection film was etched to have a predetermined patternfor formation of a pit or cavity or recess, thereby forming a mask foretching of the Si wafer 51. Thereafter, wet etching was conducted byusing the mask to form a pit or cavity or recess of 20 μm in depth asshown in FIG. 15. The etching was carried out by using 5 wt % KOH watersolution as etching fluid at a fluid temperature of 70° C.Alternatively, the depth of the pit may be set to 3 μm.

Thereafter, a SiO₂ layer 53 was formed on the surface of the wafer 51again by thermal oxidation, thereby achieving the structure that the pit52 was formed on the substrate comprising the Si wafer 51 and the SiO₂layer 53.

Thereafter, a Cr film was formed on the surface (upper surface) of thesubstrate, and then subjected to a pattern etching treatment so thatonly a portion of the Cr film that surrounds the pit 52 is left in theannular form, whereby an adhesion electrode layer or adherence electrodelayer 161 comprising the Cr film was formed so as to surround the pit52. The formation of the Cr film was carried out by a DC magnetronsputtering method under the condition that Ar was used as sputter gasand the substrate temperature was set to the room temperature. The Cradhesion electrode layer 161 was formed so that the area (S1) of theface of the upper surface thereof serving as the contact face with thelower electrode layer was equal to 4500 μm² and the film thickness wasequal to 100 nm.

Thereafter, as shown in FIG. 17, PSG was deposited on the SiO₂ layer 53having the pit 52 formed thereon and the Cr adhesion electrode layer 61at 450° C. by using silane and phosphine (PH₃). Alternatively, the PSGlayer thus deposited may be subjected to heat treatment at 850° C. for20 minutes under the 1% oxygen/nitrogen mixture atmosphere to bereflowed, thereby increasing the hardness thereof.

Subsequently, as shown in FIG. 18, the surface of PSG layer thusdeposited was polished to remove a portion of the PSG layer on theadhesion electrode layer 161, the surface of the PSG layer 55 waspolished by using slurry containing fine polishing particles and thenthe surface of the Cr adhesion electrode layer 161 was cleaned by anreverse sputtering treatment. As a result, the surface of the PSGsacrificial layer 55 was treated to have such surface roughness that theRMS variation of the height was equal to 8 nm.

Thereafter, as shown in FIG. 19, a lower electrode layer 162 of Au wasformed on the Cr adhesion electrode layer 161 and the PSG sacrificiallayer 55. The lift-off method was used for the patterning of the lowerelectrode layer 162 to achieve the lower electrode layer 162 having apredetermined shape with an outer peripheral edge corresponding to theouter peripheral edge of the Cr adhesion electrode layer 161. Theformation of Au film was conducted by using the DC magnetron sputteringmethod under the condition that Ar was used as sputter gas and thesubstrate temperature was set to the room temperature. The lowerelectrode layer 162 was formed so that the plane area (S2) was equal to27225 μm² and the film thickness was equal to 100 nm. The surfaceroughness of the Au film achieved was examined, so that the RMSvariation of the height was equal to 7 nm.

Subsequently, the PSG sacrificial layer remaining around the lowerelectrode layer 162 was removed, and the piezoelectric thin film layer163 of ZnO was formed on the lower electrode layer 162. The formation ofthe ZnO film was carried out by using the RF magnetron sputtering methodunder the condition that ZnO was used as sputtering target, Ar—O₂mixture gas in which a ratio of Ar and O₂ was equal to 9:1 was used assputtering gas, the sputter gas pressure was equal to 5 mTorr and thesubstrate temperature was set to 400° C. The thickness of the ZnO filmwas equal to 1.0 μm. The surface roughness of the ZnO film was examined,so that the RMS variation of the height was equal to 4 nm.

Subsequently, the ZnO piezoelectric thin film layer 163 was subjected tothe wet etching to have a predetermined pattern having an outerperipheral edge which corresponds to the outer peripheral edge of the Cradhesion electrode layer 161 and the outer peripheral edge of the lowerelectrode layer 162 except for an opening portion required to draw out acoupling electrode. Then, an upper electrode layer 164 of Au was formedon the ZnO piezoelectric thin film 163. The upper electrode layer 164 ispatterned into a predetermined shape by the lift-off method so that theouter peripheral edge thereof is located at a more internal side thanthe inner peripheral edge of the Cr contact electrode layer 161 (seeFIG. 21). The formation of the Au film was conducted by using the DCmagnetron sputtering method under the condition that Ar was used assputter gas and the substrate temperature was set to the roomtemperature. The thickness of the Au film was set to 100 nm.

Subsequently, a small hole penetrating through the upper electrode layer164, the piezoelectric thin film layer 163 and the lower electrode layer162 was formed by RIE (Reactive Ion Etching) so as to extend downwardlyfrom the peripheral portion of the upper electrode layer 164, andetching with dilute H₂O:HF solution was conducted to remove the PSGsacrificial layer 55, thereby forming a structure that the sandwichstructure 60 of Cr/Au/ZnO/Au was suspended over the pit 52 as shown inFIG. 20. A peeling test using a scotch tape was conducted on thesandwich structure 60 thus achieved, so that no peeling was observedbetween the sandwich structure 60 and the substrate.

Further, the thin film XRD analysis was conducted on the ZnOpiezoelectric thin film layer 163 achieved, so that the c-axis of thefilm was inclined at an angle of 88.6 degrees with respect to the filmsurface. The orientation was examined on the basis of the rocking curve,so that the full width at half maximum (FWHM) of the peak of (0002) wasequal to 2.3 degrees and excellent orientation was exhibited.

Further, with respect to the thin film bulk acoustic resonator thusachieved as shown in FIGS. 20 and 21, the impedance characteristicsbetween the upper electrode layer 164 and the lower electrode layer 162and adhesion electrode layer 161 was measured by using the microwaveprober, the resonant frequency f_(r) and the antiresonant frequencyf_(a) were measured, and the electromechanical coupling coefficientk_(t) ² was calculated on the basis of these measurement values. In thiscase, “spurious” was not excited, the electromechanical couplingcoefficient k_(t) ² was equal to 5.5%, and the acoustic quality factorwas equal to 1145. The construction and adhesion strength of FBARachieved in EXAMPLE 7 and the characteristics thereof as the acousticresonator are shown in Table 2.

EXAMPLE 8

A thin film bulk acoustic resonator was manufactured in the same manneras EXAMPLE 7 except that Ti was used for the adhesion electrode layer161 in place of Cr. The formation of the Ti film was carried out byusing the DC magnetron sputtering method under the condition that Ti wasused as sputtering target, Ar was used as sputter gas and the substratetemperature was set to the room temperature. The thickness of the Tifilm was equal to 20 nm. The surface roughness of the ZnO piezoelectricthin film layer 163 was examined, so that the RMS variation of theheight was equal to 9 nm. The peeling test using a scotch tape wasconducted, so that no peeling was observed between the substrate and thesandwich structure 60. Further, the thin film XRD analysis wasconducted, so that the c-axis of the ZnO piezoelectric thin film layer163 was inclined at an angle of 89.2 degrees with respect to the filmsurface. Further, the orientation was examined on the basis of therocking curve, so that the full width at half maximum of the peak wasequal to 2.1 degrees and excellent orientation was exhibited.

In the thin film bulk acoustic resonator thus achieved, there was no“spurious” excitation, the electromechanical coupling coefficient k_(t)² was equal to 5.9% and the acoustic quality factor was equal to 772.The construction and the adhesion strength of FBAR achieved in EXAMPLE 8and the characteristics thereof as the acoustic resonator are shown inTable 2.

EXAMPLE 9

A thin film bulk acoustic resonator was manufactured in the same manneras EXAMPLE 7 except that in place of Au, Pt was used for the lowerelectrode layer 162 and the upper electrode layer 164, and the thicknessof the Cr adhesion electrode layer 161 was equal to 60 nm. The formationof the Pt film was carried out by using the DC magnetron sputteringmethod under the condition that Pt was used as sputtering target, Ar wasused as sputter gas and the substrate temperature was set to the roomtemperature. The film thickness of the Pt film was set to 100 nm. Thesurface roughness of the ZnO piezoelectric thin film layer 163 wasexamined, so that the RMS variation of the height was equal to 6 nm.Further, the peeling test using a scotch tape was conducted, so that nopeeling was observed between the substrate and the sandwich structure60. Further, the thin film XRD analysis was conducted, so that thec-axis of the ZnO piezoelectric thin film layer 163 was inclined at anangle of 88.8 degrees with respect to the film surface. Further, theorientation was examined on the basis of the rocking curve, so that thefull width at half maximum of the peak was equal to 2.5 degrees and thusexcellent orientation was exhibited.

In the thin film bulk acoustic resonator thus achieved, there was no“spurious” excitation, the electromechanical coupling coefficient k_(t)² was equal to 5.2% and the acoustic quality factor was equal to 898.The construction and the adhesion strength of FBAR achieved in EXAMPLE 9and the characteristics thereof as the acoustic resonator are shown inTable 2.

EXAMPLE 10

A thin film bulk acoustic resonator was manufactured in the same manneras EXAMPLE 7 except that in place of Cr, Ni was used for the adhesionelectrode layer 161 and the plane area S1 thereof was enlarged to 15000μm² and the ratio S1/S2 (S2: the plane area of the lower electrode layer162) was set to 0.55. The formation of the Ni film was carried out byusing the DC magnetron sputtering method under the condition that Ni wasused as sputtering target, Ar was used as sputter gas and the substratetemperature was set to the room temperature. The thickness of the Nifilm was set to 50 nm. The surface roughness of the ZnO piezoelectricthin film layer 163 was examined, so that the RMS variation of theheight was equal to 11 nm. Further, the peeling test using a scotch tapewas conducted, so that no peeling was observed between the substrate andthe sandwich structure 60. Further, the thin film XRD analysis wasconducted, so that the c-axis of the ZnO piezoelectric thin film layer163 was inclined at an angle of 89.0 degrees with respect to the filmsurface. Further, the orientation was examined on the basis of therocking curve, so that the full width at half maximum of the peak wasequal to 2.9 degrees and thus excellent orientation was exhibited.

In the thin film bulk acoustic resonator thus achieved, there was no“spurious” excitation, the electromechanical coupling coefficient k_(t)² was equal to 4.8% and the acoustic quality factor was equal to 707.The construction and the adhesion strength of FBAR achieved in EXAMPLE10 and the characteristics thereof as the acoustic resonator are shownin Table 2.

EXAMPLE 11

A thin film bulk acoustic resonator was manufactured in the same manneras EXAMPLE 8 except that in place of Au, Pt was used for the lowerelectrode layer 162 and the upper electrode layer 164, in place of ZnO,AlN was used for the piezoelectric thin film layer 163, and the planearea S1 of the Ti adhesion electrode layer 161 was set to 4000 μm² andthe thickness thereof was equal to 30 nm. The formation of the Pt filmwas carried out in the same manner as EXAMPLE 9. The formation of theAlN film was carried out by using the RF magnetron sputtering methodunder the condition that Al was used as sputtering target, Ar—N₂ mixturegas (Ar:N₂=1:1) was used as sputter gas and the substrate temperaturewas set to 400° C. The thickness of the AlN film was set to 1.4 μm. Thesurface roughness of the AlN film thus achieved was examined, so thatthe RMS variation of the height was equal to 7 nm. Further, the peelingtest using a scotch tape was conducted, so that no peeling was observedbetween the substrate and the sandwich structure 60. Further, the thinfilm XRD analysis was conducted, so that the c-axis of the AlNpiezoelectric thin film layer 163 was inclined at an angle of 90.0degrees with respect to the film surface. Further, the orientation wasexamined on the basis of the rocking curve, so that the full width athalf maximum of the peak was equal to 2.7 degrees and thus excellentorientation was exhibited.

In the thin film bulk acoustic resonator thus achieved, there was no“spurious” excitation, the electromechanical coupling coefficient k_(t)² was equal to 6.4% and the acoustic quality factor was equal to 984.The construction and the adhesion strength of FBAR achieved in EXAMPLE11 and the characteristics thereof as the acoustic resonator are shownin Table 2.

EXAMPLE 12

A thin film bulk acoustic resonator was manufactured in the same manneras EXAMPLE 11 except that Cr was used for the adhesion electrode layer161, Mo was used for the upper and lower electrode layers 162, 164, andthe plane area S1 of the Cr adhesion electrode layer 161 was set to 5000μm² and the thickness thereof was set to 40 nm. The surface roughness ofthe AlN film thus achieved was examined, so that the RMS variation ofthe height was equal to 5 nm. Further, the peeling test using a scotchtape was conducted, so that no peeling was observed between thesubstrate and the sandwich structure 60. Further, the thin film XRDanalysis was conducted, so that the c-axis of the AlN piezoelectric thinfilm layer 163 was inclined at an angle of 89.8 degrees with respect tothe film surface. Further, the orientation was examined on the basis ofthe rocking curve, so that the full width at half maximum of the peakwas equal to 2.9 degrees and thus excellent orientation was exhibited.

In the thin film bulk acoustic resonator thus achieved, there was no“spurious” excitation, the electromechanical coupling coefficient k_(t)² was equal to 6.1% and the acoustic quality factor was equal to 1140.The construction and the adhesion strength of FBAR achieved in EXAMPLE12 and the characteristics thereof as the acoustic resonator are shownin Table 2.

COMPARATIVE EXAMPLE 4

A thin film bulk acoustic resonator was manufactured in the same manneras EXAMPLE 7 except that PSG was deposited on the structure that a pit52 was formed on a substrate comprising Si wafer 51 and a SiO₂ layer 53,the surface of the PSG layer thus formed was polished to remove aportion of the PSG layer in the area out of the recess 52, the surfaceof the PSG layer in the area of the pit 52 was treated to have suchsurface roughness that the RMS variation of the height was equal to 38nm, a Cr film and an Au film were formed on the PSG layer and thesefilms were patterned in the same pattern to achieve the structure thatthe adhesion electrode layer 161 was joined to the whole surface of thelower electrode layer 162.

The surface roughness of the ZnO film thus achieved was examined, sothat the RMS variation of the height was equal to 30 nm. Further, thepeeling test using a scotch tape was conducted, so that no peeling wasobserved between the substrate and the sandwich structure 60. Further,the thin film XRD analysis was conducted, so that the c-axis of the ZnOpiezoelectric thin film layer 163 was inclined at an angle of 87.5degrees with respect to the film surface. Further, the orientation wasexamined on the basis of the rocking curve, so that the full width athalf maximum of the peak was equal to 4.8 degrees, which indicated thatthe orientation was more degraded by 2.5 degrees than that of EXAMPLE 7.

In the thin film bulk acoustic resonator thus achieved, “spurious” wasexcited, the electromechanical coupling coefficient k_(t) ² was equal to2.5% and the acoustic quality factor was equal to 404. The constructionand the adhesion strength of FBAR achieved in COMPARATIVE EXAMPLE 4 andthe characteristics thereof as the acoustic resonator are shown in Table2.

COMPARATIVE EXAMPLE 5

A thin film bulk acoustic resonator was manufactured in the same manneras COMPARATIVE EXAMPLE 4 except that no adhesion electrode layer 161 wasprovided. However, the surface of the PSG layer in the area of the pit52 was treated so that the RMS variation of the height was equal to 33nm.

The surface roughness of the ZnO film thus achieved was examined, sothat the RMS variation of the height was equal to 23 nm. The thin filmXRD analysis was conducted, so that the c-axis of the ZnO piezoelectricthin film layer 163 was inclined at an angle of 88.4 degrees withrespect to the film surface. The orientation was examined on the basisof the rocking curve, so that the full width at half maximum of the peakwas equal to 4.2 degrees. Further, in the peeling test using a scotchtape, peeling was observed between the substrate and the sandwichstructure 60.

In the thin film bulk acoustic resonator thus achieved, “spurious” wasexcited, the electromechanical coupling coefficient k_(t) ² was equal to3.2 and the acoustic quality factor was equal to 446. The constructionand the adhesion strength of FBAR achieved in COMPARATIVE EXAMPLE 5 andthe characteristics thereof as the acoustic resonator are shown in Table2.

EXAMPLES 13 to 15

Piezoelectric thin film resonators each having the structure shown inFIGS. 22 and 23 were manufactured as follows.

A silicon oxide (SiO₂) layer of 0.3 to 0.6 μm in thickness was formed oneach of the upper surface and lower surface of a (100) Si substrate 112of 250 μm in thickness by the thermal oxidation method. The SiO₂ layerat the upper surface side was used as the insulating layer 13. Further,the SiO₂ layer at the lower surface side was treated to have a maskpattern for forming a via hole for the substrate 112 as described later.

A Mo layer of 0.1 μm in thickness was formed on the surface of theinsulating layer 13 by the DC magnetron sputtering method, and patternedby the photolithography technique to thereby form a lower electrode 15.The main body portion 15 a of the lower electrode 15 was designed in anapproximately rectangular shape having a plane size of 140×160 μm.Further, an AlN thin film of 1.3 to 2.0 μm in thickness, the crystalface of which was oriented in the C-axis, was formed on the Mo lowerelectrode 15. The formation of the AlN thin film was conducted by thereactive RF magnetron sputtering method. The AlN thin film was patternedinto a predetermined shape by a wet etching treatment using heatedphosphoric acid to form a piezoelectric film 16. Thereafter, an upperelectrode 17 formed of Mo and having a thickness of 0.1 μm was formed byusing the DC magnetron sputtering method and the lift-off method. Themain body portion 17 a of the upper electrode 17 was designed in anapproximately rectangular shape having a plane size of 140×160 μm anddisposed at the position corresponding to the main body portion 15 a ofthe lower electrode.

Subsequently, the side of the above achieved structure with the upperand lower electrodes 15, 17 and the piezoelectric film 16 was coatedwith PMMA resin, and the portion of the Si substrate 112 correspondingto the vibrating portion 121 was etched away with KOH water solution byusing as a mask the patterned SiO₂ layer formed on the lower surface ofthe Si substrate 112, thereby forming a via hole 120 serving as acavity. The size of the opening of the via hole formed on the uppersurface of the Si substrate 112 (the plane size of the vibrating portion21) was set to 200×200 μm.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the impedance between the electrode terminals 15b, 17 b of the thin film piezoelectric resonator was measured by using amicrowave prober produced by Cascade Microtech Incorporated Company anda network analyzer, and the electromechanical coupling coefficient k_(t)², the frequency temperature characteristic τ_(f), and the acousticquality factor Q were determined on the basis of the measurement valuesof the resonant frequency f_(r) and the antiresonant frequency f_(a).The fundamental frequency, the electromechanical coupling coefficientk_(t) ², the frequency temperature characteristic τ_(f) and the acousticquality factor Q of the thickness vibration of the piezoelectric thinfilm resonator achieved are shown in Table 3.

EXAMPLES 16 to 18

Piezoelectric thin film resonators each having the structure shown inFIGS. 24 and 25 were manufactured as follows.

That is, the same process as EXAMPLES 13 to 15 was carried out exceptthat after the upper electrode 17 was formed and before the via hole 120was formed, a SiO₂ layer of 0.1 to 0.3 μm was formed on the upperelectrode 17 by the RF magnetron sputtering method and patterned inconformity with the vibrating portion 121 to form an upper insulatinglayer 18, and also the thickness of the lower insulating layer 13 andthe thickness of the piezoelectric film 16 were set as shown in Table 3.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the fundamental frequency of the thicknessvibration, the electromechanical coupling coefficient k_(t) ², thefrequency temperature characteristic τ_(f) and the acoustic qualityfactor Q were determined in the same manner as EXAMPLES 13 to 15, andthese data are shown in Table 3.

EXAMPLES 19 to 22

Piezoelectric thin film resonators each having the structure shown inFIGS. 26 and 27 and piezoelectric thin film resonators each having thestructure shown in FIGS. 28 and 29 were manufactured as follows.

That is, the same process [for EXAMPLES 19, 20] as EXAMPLES 13 to 15 andthe same process [for EXAMPLES 21, 22] as EXAMPLES 16 to 18 were carriedout except that the thickness of the upper and lower insulating layers13, 18 and the thickness of the piezoelectric film 16 were set as shownin Table 3 and also except for the shape and size of the upper and lowerelectrodes 15, 17. The lower electrode 15 was designed in a rectangularshape having a plane size of 150×300 μm extending so as to contain thearea corresponding to the vibrating portion 121, and the upper electrode17 were so designed that main body portions 17Aa, 17Ba having anapproximately rectangular shape of 70×90 μm in plane size were disposedso as to be spaced from each other at a distance of 20 μm.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the fundamental frequency of the thicknessvibration, the electromechanical coupling coefficient k_(t) ², thefrequency temperature characteristic τ_(f) and the acoustic qualityfactor Q were determined in the same manner as EXAMPLES 13 to 15 andEXAMPLES 16 to 18, and these data are shown in Table 3.

EXAMPLES 23 to 25

Piezoelectric thin film resonators each having the structure shown inFIGS. 22 and 23 and piezoelectric thin film resonators each having thestructure shown in FIGS. 24 and 25 were manufactured as follows.

That is, the same process [for EXAMPLES 23, 24] as EXAMPLE 13 and thesame process [for EXAMPLES 25] as EXAMPLE 16 were carried out exceptthat the thickness of the piezoelectric film 16 and the thickness of theupper and lower insulating layers 13, 18 were set as shown in Table 3.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the fundamental frequency of the thicknessvibration, the electromechanical coupling coefficient k_(t) ², thefrequency temperature characteristic τ_(f) and the acoustic qualityfactor Q were determined in the same manner as EXAMPLES 13 and EXAMPLE16, and these data are shown in Table 3.

COMPARATIVE EXAMPLES 6 and 7

The same process as EXAMPLE 13 was conducted except that in place of Mo,aluminum (Al) was used as the material of the upper and lower electrodelayers, and the thickness of the piezoelectric layer and the thicknessof the insulating layer 13 were set as shown in Table 3.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the fundamental frequency of the thicknessvibration, the electromechanical coupling coefficient k_(t) ², thefrequency temperature characteristic τ_(f) and the acoustic qualityfactor Q were determined in the same manner as EXAMPLE 13, and thesedata are shown in Table 3.

COMPARATIVE EXAMPLE 8

The same process as EXAMPLE 13 was carried out except that theinsulating layer 13 was left only out of the area corresponding to thevibrating portion 121.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the fundamental frequency of the thicknessvibration, the electromechanical coupling coefficient k_(t) ², thefrequency temperature characteristic τ_(f) and the acoustic qualityfactor Q were determined in the same manner as EXAMPLE 13, and thesedata are shown in Table 3.

COMPARATIVE EXAMPLES 9 and 10

The same process as EXAMPLE 13 was carried out except that in place ofAlN, zinc oxide (ZnO) was used as the material of the piezoelectric film16, and the thickness of the piezoelectric film 16 and the thickness ofthe insulating layer 13 were set as shown in Table 3.

With respect to the thin film piezoelectric resonators (FBARs) achievedin the above process, the fundamental frequency of the thicknessvibration, the electromechanical coupling coefficient k_(t) ², thefrequency temperature characteristic τ_(f) and the acoustic qualityfactor Q were determined in the same manner as EXAMPLE 13, and thesedata are shown in Table 3.

From the above-described results, there can be achieved thepiezoelectric thin film resonator having excellent temperature stabilityof the resonant frequency with keeping the electromechanical couplingcoefficient and the acoustic quality factor by joining and adding to thepiezoelectric stack structure the insulating layer formed of mainlysilicon oxide having a temperature coefficient whose sign is differentfrom that of the resonant frequency of the vibrating portion containinga part of the piezoelectric stack structure comprising the electrodesformed of mainly molybdenum and the piezoelectric film formed of mainlyaluminum nitride. Particularly, when it is applied to VCO (piezoelectricthin film resonator), a filter or a transmission/reception duplexer in ahigh frequency band of 1 GHz or more, the performance thereof can beremarkably improved.

TABLE 1 CONSTRUCTION OF FBAR CHARACTERISTIC AS INSULATING ACOUSTICRESONATOR LAYER LOWER PIEZOELECTRIC LAYER ELECTRO- ACOUS- SUR- ELECTRODESUR- ROCK- MECHANICAL TIC FACE SURFACE FACE c-AXIS ING UPPER ELECTRODECOUPLING QUAL- ROUGH- ROUGH- ROUGH- DIREC- CURVE WAVINESS COEFFI- ITYMATE- NESS MATE- NESS MATE- NESS TION FWHM MATE- HEIGHT CIENT FACTORRIAL RMS (nm) RIAL RMS (nm) RIAL RMS (nm) (deg) (deg) RIAL (nm) k_(t) ²(%) Q EX. 1 none — Mo 15 ZnO 11 88.5 2.5 Mo 200 5.5 700 EX. 2 none — Mo15 AlN 14 88.5 2.8 Mo 200 6.5 900 EX. 3 SiO₂ 10 Mo 15 ZnO 10 88.5 2.3 Mo200 4.5 650 EX. 4 none — Mo 15 AlN 14 88.5 2.8 Mo 150 7.5 950 EX. 5 none— Mo 13 AlN 10 89.5 2.2 Mo 150 6.7 980 EX. 6 Si₃N₄ 12 Mo 17 AlN 15 88.42.8 Mo 210 5.2 700 COMP. none — Mo 80 ZnO 75 85.0 7.0 Mo 1000 3.0 400EX. 1 COMP. none — Mo 85 AlN 80 83.0 8.5 Mo 1250 3.5 450 EX. 2 COMP.SiO₂ 85 Mo 90 ZnO 85 83.0 9.5 Mo 1000 2.8 360 EX. 3

TABLE 2 CHARACTERISTIC AS CONSTRUCTION OF FBAR ACOUSTIC LOWER RESONATORADHESION ELECTRODE PIEZOELECTRIC THIN MATE- ELECTRO- ELECTRODE LAYERFILM LAYER RIAL MECHAN- ACOUS- LAYER SUR- SUR- ROCK- OF ICAL TIC PLANEFACE FACE c-AXIS ING UPPER ADHESION COUPLING QUAL- AREA THICK- ROUGH-ROUGH- DIREC- CURVE ELEC- STRENGTH COEFFI- ITY MATE- S1 NESS MATE- NESSMATE- NESS TION FWHM TRODE PEELING CIENT FACTOR RIAL (μm²) (nm) RIAL RMS(nm) RIAL RMS (nm) (deg) (deg) LAYER TEST k_(t) ² (%) Q EX. 7 Cr 4,500100 Au 7 ZnO 4 88.6 2.3 Au NOT 5.5 1145 PEELED EX. 8 Ti 4,500 20 Au 10ZnO 9 89.2 2.1 Au NOT 5.9 772 PEELED EX. 9 Cr 4,500 60 Pt 9 ZnO 6 88.82.5 Pt NOT 5.2 898 PEELED EX. 10 Ni 15,000 50 Au 12 ZnO 11 89.0 2.9 AuNOT 4.8 707 PEELED EX. 11 Ti 4,000 30 Pt 9 AlN 7 90.0 2.7 Pt NOT 6.4 984PEELED EX. 12 Cr 5,000 40 Mo 8 AlN 5 89.8 2.9 Mo NOT 6.1 1140 PEELEDCOMP. Cr 27,225 20 Au 40 ZnO 30 87.5 4.8 Au NOT 2.5 404 EX. 4 PEELEDCOMP. — — — Au 28 ZnO 23 88.4 4.2 Au PEELED 3.2 446 EX. 5

TABLE 3 ELECTRO- ANTI- MECHAN- THICKNESS RESO- ICAL CONSTRUC- OF PIEZO-THICKNESS OF THICK- NANT COUPLING FREQUENCY ACOUSTIC TION ELECTRICINSULATING NESS RESONANT FRE- COEFFI- TEMPERATURE QUALITY OF FBAR- THINFILM LAYER (μm) RATIO FREQUENCY QUENCY CIENT COEFFICIENT FACTOR FIG. NO.(μm) LOWER UPPER t′/t f_(r) (GHz) f_(a) (GHz) k_(t) ² (%) τ_(f) (ppm/°C.) Q EX. 13 22, 23 1.60 0.50 — 0.31 2.055 2.093 4.4 3.7 1640 EX. 14 22,23 2.00 0.30 — 0.15 2.049 2.090 4.8 −13.4 1720 EX. 15 22, 23 1.30 0.60 —0.46 2.118 2.155 4.2 14.5 1120 EX. 16 24, 25 1.60 0.30 0.20 0.31 2.0302.076 5.3 4.2 1610 EX. 17 24, 25 1.90 0.30 0.10 0.21 2.005 2.051 5.5−5.6 1800 EX. 18 24, 25 1.30 0.30 0.30 0.46 2.158 2.202 4.9 14.2 1060EX. 19 26, 27 1.90 0.40 — 0.21 1.984 2.024 4.7 −3.9 1670 EX. 20 26, 271.30 0.60 — 0.46 2.141 2.179 4.3 14.4 980 EX. 21 28, 29 1.90 0.30 0.100.21 1.998 2.043 5.4 −5.1 1650 EX. 22 28, 29 1.30 0.30 0.30 0.46 2.1182.163 5.1 14.1 940 EX. 23 22, 23 1.00 0.53 — 0.53 2.525 2.568 4.1 18.5800 EX. 24 22, 23 2.20 0.19 — 0.08 2.077 2.124 5.5 −18.3 1720 EX. 25 24,25 2.10 0.10 0.08 0.08 2.091 2.144 6.1 −18.1 1880 COMP. 22, 23 1.10 0.60— 0.55 2.262 2.293 3.3 24.0 320 EX. 6 COMP. 22, 23 1.60 0.50 — 0.311.774 1.800 3.5 −20.5 550 EX. 7 COMP. 22, 23 2.20 — — — 2.318 2.375 5.8−35.0 1750 EX. 8 COMP. 22, 23 1.90 0.16 — 0.08 2.136 2.171 3.9 −47.4 750EX. 9 COMP. 22, 23 0.90 0.80 — 0.89 1.748 1.768 2.8 23.7 210 EX. 10

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the firstelectrode is formed directly or through the insulating layer on thesurface of the sacrificial layer that is smooth at an atomic level sothat the RMS variation of the height is not more than 25 nm, preferablynot more than 20 nm, whereby the RMS variation of the height of thesurface of the first electrode is set to 25 nm or less, preferably to 20nm or less, and the piezoelectric layer is formed on the surface of thefirst electrode. Therefore, the crystallinity of the first electrode isimproved, and thus the orientation and crystal quality of thepiezoelectric layer can be remarkably improved. Accordingly, there canbe provided a high-performance thin film bulk acoustic resonator that isexcellent in electromechanical coupling coefficient and acoustic qualityfactor.

Further, as described above, according to the present invention, theadhesion electrode layer is provided between the lower electrode layerand the substrate, and the adhesion electrode layer is joined to thesubstrate around the pit formed in the substrate. Therefore, occurrenceof vibration in the lateral direction in the thin film bulk acousticresonator can be suppressed, extra “spurious” can be prevented frombeing superposed on the vibration of the thin film bulk acousticresonator, so that the resonance characteristic and the quality factorof the thin film bulk resonator and filter can be improved. Further, noadhesion electrode layer exists at the lower side of the center portionof the lower electrode layer (that is, the inside portion surrounded bythe adhesion electrode layer), so that the center portion of the lowerelectrode layer can be formed on the surface of the sacrificial layerhaving extremely high smoothness and thus the orientation andcrystallinity can be enhanced. Accordingly, the piezoelectric thin filmlayer having excellent orientation and crystallinity can be formed, andthe high-performance thin film bulk acoustic resonator having excellentelectromechanical coupling coefficient and acoustic quality factor (Qvalue) can be provided. Still further, by using the adhesion electrodelayer, the adhesion force (bonding strength) between the lower electrodelayer and the substrate can be increased, so that the range of thematerials to be selectively available for the lower electrode layers canbe expanded, and the durability of the thin film bulk acoustic resonatorcan be improved to extend the lifetime thereof.

Further, as described above, according to the piezoelectric thin filmresonator of the present invention, the electrodes formed mainly ofmolybdenum, the piezoelectric film formed mainly of aluminum nitride andthe insulating layer formed mainly of silicon oxide or silicon nitrideare used in combination, so that the electromechanical couplingcoefficient, the acoustic quality factor (Q value) and the frequencytemperature characteristic can be improved.

1. A thin film bulk acoustic resonator, comprising: a piezoelectriclayer; a first electrode joined to a first surface of said piezoelectriclayer; and a second electrode joined to a second surface of saidpiezoelectric layer, which is located at the opposite side to the firstsurface; wherein RMS variation of the height of the first surface ofsaid piezoelectric layer is equal to 25 nm or less, and wherein the RMSvariation of the height of the second surface of said piezoelectriclayer is set to not more than 5% of the thickness of said piezoelectriclayer.
 2. A thin film bulk acoustic resonator, comprising: apiezoelectric layer; a first electrode joined to a first surface of saidpiezoelectric layer; and a second electrode joined to a second surfaceof said piezoelectric layer, which is located at the opposite side tothe first surface; wherein RMS variation of the height of the firstsurface of said piezoelectric layer is equal to 25 nm or less, andwherein a waviness height of a surface of said second electrode is setto not more than 25% of the thickness of said piezoelectric layer.
 3. Athin film bulk acoustic resonator, comprising: a piezoelectric layer; afirst electrode joined to a first surface of said piezoelectric layer;and a second electrode joined to a second surface of said piezoelectriclayer, which is located at the opposite side to the first surface;wherein RMS variation of the height of the first surface of saidpiezoelectric layer is equal to 25 nm or less, and wherein said secondelectrode has a center portion and an outer peripheral portion having alarger thickness than said center portion.
 4. The thin film bulkacoustic resonator as claimed in any one of claims 1, 2 or 3, whereinthe RMS variation of the height of the first surface of saidpiezoelectric layer is equal to 20 nm or less.
 5. The thin film bulkacoustic resonator as claimed in claim 3, wherein the waviness height ofa surface of said center portion is set to not more than 25% of thethickness of said piezoelectric layer.
 6. The thin film bulk acousticresonator as claimed in claim 3, wherein said outer peripheral portionis located in a frame shape so as to surround said center portion. 7.The thin film bulk acoustic resonator as claimed in claim 3, whereinsaid second electrode is designed so that thickness variation of saidcenter portion is set to not more than 1% of the thickness of saidcenter portion.
 8. The thin film bulk acoustic resonator as claimed inclaim 3, wherein the thickness of said outer peripheral portion is setto not less than 1.1 time as high as the height of said center portion.9. The thin film bulk acoustic resonator as claimed in claim 3, whereinsaid outer peripheral portion is located within an area inwardlyextending from an outer edge of said second electrode by a distance of40 μm.
 10. A thin film bulk acoustic resonator, comprising: apiezoelectric layer; a first electrode joined to a first surface of saidpiezoelectric layer; a second electrode joined to a second surface ofsaid piezoelectric layer, which is located at the opposite side to thefirst surface; and an insulating layer formed on a surface of asubstrate so as to stride over a pit portion formed on the surface ofsaid substrate, wherein a sandwich structure comprising saidpiezoelectric layer, said first electrode and said second electrode issupported at an edge portion thereof by said substrate so as to strideover said pit portion, said sandwich structure is formed on theinsulating layer, and RMS variation of the height of the first surfaceof said piezoelectric layer is equal to 25 nm or less.
 11. A thin filmbulk acoustic resonator, comprising: a piezoelectric layer; a firstelectrode joined to a first surface of said piezoelectric layer; and asecond electrode joined to a second surface of said piezoelectric layer,which is located at the opposite side to the first surface, wherein asurface of said first electrode facing said piezoelectric layer has RMSvariation of the height thereof that is equal to 25 nm or less, whereina sandwich structure comprising said piezoelectric layer, said firstelectrode and said second electrode are supported at an edge portionthereof by a substrate so as to stride over a pit portion formed on asurface of said substrate, and wherein an insulating layer is formed onthe surface of said substrate so as to stride over said pit portion, andsaid sandwich structure is formed on the insulating layer.
 12. A thinfilm bulk acoustic resonator, comprising: a piezoelectric layer; a firstelectrode joined to a first surface of said piezoelectric layer; and asecond electrode joined to a second surface of said piezoelectric layer,which is located at the opposite side to the first surface, wherein asurface of said first electrode facing said piezoelectric layer has RMSvariation of the height thereof that is equal to 25 nm or less, andwherein the RMS variation of the height of the second surface of saidpiezoelectric layer is set to not more than 5% of the thickness of saidpiezoelectric layer.
 13. A thin film bulk acoustic resonator,comprising: a piezoelectric layer; a first electrode joined to a firstsurface of said piezoelectric layer; and a second electrode joined to asecond surface of said piezoelectric layer, which is located at theopposite side to the first surface, wherein a surface of said firstelectrode facing said piezoelectric layer has RMS variation of theheight thereof that is equal to 25 nm or less, and wherein a wavinessheight of a surface of said second electrode is set to not more than 25%of the thickness of said piezoelectric layer.
 14. A thin film bulkacoustic resonator, comprising: a piezoelectric layer; a first electrodejoined to a first surface of said piezoelectric layer; and a secondelectrode joined to a second surface of said piezoelectric layer, whichis located at the opposite side to the first surface, wherein a surfaceof said first electrode facing said piezoelectric layer has RMSvariation of the height thereof that is equal to 25 nm or less, andwherein said second electrode has a center portion and an outerperipheral portion having a larger thickness than said center portion.15. The thin film bulk acoustic resonator as claimed in any one ofclaims 12, 13 or 14, wherein the surface of said first electrode facingsaid piezoelectric layer has the RMS variation of 20 nm or less.
 16. Thethin film bulk acoustic resonator as claimed in claim 14, wherein thewaviness height of a surface of said center portion is set to not morethan 25% of the thickness of said piezoelectric layer.
 17. The thin filmbulk acoustic resonator as claimed in claim 14, wherein said outerperipheral portion is located within an area inwardly extending from anouter edge of said second electrode by a distance of 40 μm.
 18. The thinfilm bulk acoustic resonator as claimed in claim 14, wherein said outerperipheral portion is located in a frame shape so as to surround saidcenter portion.
 19. The thin film bulk acoustic resonator as claimed inclaim 14, wherein said second electrode is designed so that thicknessvariation of said center portion is set to not more than 1% of thethickness of said center portion.
 20. The thin film bulk acousticresonator as claimed in claim 14, wherein the thickness of said outerperipheral portion is set to not less than 1.1 time as high as theheight of said center portion.
 21. A thin film bulk acoustic resonator,comprising: a piezoelectric layer formed of AlN or ZnO; a firstelectrode joined to a first surface of said piezoelectric layer andformed of material containing at least one material selected from agroup consisting of Au, Pt, W and Mo; and a second electrode joined to asecond surface of said piezoelectric layer, which is located at theopposite side to the first surface; wherein RMS variation of the heightof the first surface of said piezoelectric layer is equal to 25 nm orless, and a full width at half maximum of XRD rocking curve of saidpiezoelectric layer is 2.2 to 2.8 degrees.
 22. A thin film bulk acousticresonator, comprising: a piezoelectric layer formed of AlN or ZnO; afirst electrode joined to a first surface of said piezoelectric layerand formed of material containing at least one material selected fromthe group consisting of Au, Pt, W and Mo; and a second electrode joinedto a second surface of said piezoelectric layer, which is located at theopposite side to the first surface; wherein RMS variation of the heightof the first surface of said piezoelectric layer is equal to 25 nm orless, and an electromechanical coupling coefficient determined on thebasis of measured values of resonance frequency and antiresonancefrequency of approximately 2.0 GHz is equal to 4.5 to 7.5%.
 23. A thinfilm bulk acoustic resonator, comprising: a piezoelectric layer formedof AlN or ZnO; a first electrode joined to a first surface of saidpiezoelectric layer and formed of material containing at least onematerial selected from the group consisting of Au, Pt, W and Mo; and asecond electrode joined to a second surface of said piezoelectric layer,which is located at the opposite side to the first surface; wherein RMSvariation of the height of the first surface of said piezoelectric layeris equal to 25 nm or less, and an acoustic quality factor determined onthe basis of measured values of resonance frequency and antiresonancefrequency of approximately 2.0 GHz is equal to 650 to
 980. 24. A thinfilm bulk acoustic resonator, comprising: a piezoelectric layer formedof AlN or ZnO; a first electrode joined to a first surface of saidpiezoelectric layer and formed of material containing at least onematerial selected from the group consisting of Au, Pt, W and Mo; and asecond electrode joined to a second surface of said piezoelectric layer,which is located at the opposite side to the first surface; wherein asurface of said first electrode facing said piezoelectric layer has RMSvariation of the height thereof that is equal to 25 nm or less, and afull width at half maximum of XRD rocking curve of said piezoelectriclayer is 2.2 to 2.8 degrees.
 25. A thin film bulk acoustic resonator,comprising: a piezoelectric layer formed of AlN or ZnO; a firstelectrode joined to a first surface of said piezoelectric layer andformed of material containing at least one material selected from thegroup consisting of Au, Pt, W and Mo; and a second electrode joined to asecond surface of said piezoelectric layer, which is located at theopposite side to the first surface; wherein a surface of said firstelectrode facing said piezoelectric layer has RMS variation of theheight thereof that is equal to 25 nm or less, and an electromechanicalcoupling coefficient determined on the basis of measured values ofresonance frequency and antiresonance frequency of approximately 2.0 GHzis equal to 4.5 to 7.5%.
 26. A thin film bulk acoustic resonator,comprising: a piezoelectric layer formed of AlN or ZnO; a firstelectrode joined to a first surface of said piezoelectric layer andformed of material containing at least one material selected from thegroup consisting of Au, Pt, W and Mo; and a second electrode joined to asecond surface of said piezoelectric layer, which is located at theopposite side to the first surface; wherein a surface of said firstelectrode facing said piezoelectric layer has RMS variation of theheight thereof that is equal to 25 nm or less, and an acoustic qualityfactor determined on the basis of measured values of resonance frequencyand antiresonance frequency of approximately 2.0 GHz is equal to 650 to980.
 27. The thin film bulk acoustic resonator as claimed in any one ofclaims 21, 22, 23, 24, 25, or 26, wherein a sandwich structurecomprising said piezoelectric layer, said first electrode and saidsecond electrode is supported at an edge portion thereof by a substrateso as to stride over a pit portion formed on a surface of saidsubstrate.
 28. The thin film bulk acoustic resonator as claimed in claim27, wherein an insulating layer is formed on the surface of saidsubstrate so as to stride over said pit portion, and said sandwichstructure is formed on the insulating layer.